Cherry pick PR #8928: Implement basic application preload functionality.

[cobalt-github-releaser-bot]

Refer to the original PR: #8928

Implement background preloading support for instantaneous application launching.

This feature allows Cobalt to initialize in a hidden state, deferring heavy resource
creation until the application is revealed to the foreground. To further minimize
footprint, the splash screen creation is skipped entirely during preloading.

• Support starting Cobalt in a background/hidden state via the kSbEventTypePreload signal.
• Defer creation of native windows and UI widgets until the application is revealed,
  minimizing initial memory and CPU footprint.
• Skip creation of splash screen WebContents when preloaded to further reduce
  background resource usage.
• Handle kSbEventTypeReveal events to transition the application from a background
  to a visible state and initialize deferred UI components.
• Ensure standard Web APIs (document.visibilityState, document.hidden) correctly
  reflect the application's hidden state during preloading and its revealed state.
• Standardize on visibility state propagation throughout the browser initialization
  chain (MainDelegate, BrowserClient, MainParts), keeping "preload" logic encapsulated
  at the Starboard entry point.
• Consolidate window creation logic into a private internal helper to ensure
  consistent initialization for both normal and preloaded startups.
• Refactor shell unit tests into a reusable ShellTestBase and extract common mock
  infrastructure into shell_test_support.h.
• Add comprehensive unit tests:
  • LifecycleTest: Verifies startup states, revelation, and signal idempotency.
  • SplashScreenTest: Verifies that splash screen is skipped during preloading.
• Add a comprehensive automated integration test (test_preload.sh) to verify
  preloading, revelation, and clean shutdown.
• Add detailed documentation for developers, device manufacturers, and web
  designers in cobalt/doc/preload.md.
• Enhanced developer tools with usage examples and captured test output.

Bug: 447660888

[gemini-code-assist]

Code Review

This pull request is a cherry-pick that implements application preloading. This allows Cobalt to start in a hidden background state and defer resource-intensive UI initialization until the application is brought to the foreground, enabling a near-instantaneous startup experience for the user.

The changes are comprehensive and well-executed:

• A new kSbEventTypePreload event is handled to initiate a hidden startup.
• The concept of visibility is plumbed through the browser initialization stack, from CobaltMainDelegate down to the ShellPlatformDelegate.
• Platform-specific delegates (aura, views) are updated to defer native window and widget creation until a kSbEventTypeReveal event is received.
• Splash screen creation is correctly skipped during a preload.
• New unit tests (LifecycleTest) and integration tests (test_preload.sh) have been added to verify the new lifecycle states and visibility transitions.
• Existing tests (SplashScreenTest) have been updated and refactored into a common ShellTestBase for better reusability.
• Detailed documentation for the preload feature has been added.

The implementation appears robust and correct. I have reviewed the changes and found no issues. The code adheres to the project's style and structure.

[datadog-cobalt-youtube]

⚠️ Tests

✨ Fix all issues with Cursor

⚠️ Warnings

🧪 1 Test failed

UnknownTest from UnknownSuite (Datadog) (Fix with Cursor)

Test crashed

No crash detected in test log.

ℹ️ Info

❄️ No new flaky tests detected

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