Update DevTools DEPS (trusted)

Rolling build: https://chromium.googlesource.com/chromium/src/build/+log/cac7cbb..a9640b2

Roll created at https://cr-buildbucket.appspot.com/build/8724137447749707841

Bug: none
Change-Id: I140e640566fc4fb4acb016996a695de22ff341b2
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/6216128
Commit-Queue: Devtools Autoroller <devtools-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Devtools Autoroller <devtools-ci-autoroll-builder@chops-service-accounts.iam.gserviceaccount.com>

Author: devtools-ci-autoroll-builder

DEPS

@@ -12,7 +12,7 @@ vars = {
   'build_with_chromium': False,
 
   'build_url': 'https://chromium.googlesource.com/chromium/src/build.git',
-  'build_revision': 'cac7cbbb5e9027ffcb2b60f1805b66d45ec77038',
+  'build_revision': 'a9640b2af4c541cbe6b716f447315d487351ae46',
 
   'buildtools_url': 'https://chromium.googlesource.com/chromium/src/buildtools.git',
   'buildtools_revision': '6b4eaa1ed0f3a604f354b4098e4f676f7815f1da',