RubyLB: Fix a crash in ShouldBeInlinified()

This fixes a regression by crrev.com/1439223.
Parent's GetComputedStyle() can be nullptr.

Fixed: 406999114
Change-Id: I90daa8e96230a775d88a4bbd1519531a08d87d54
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6414328
Reviewed-by: Koji Ishii <[email protected]>
Commit-Queue: Koji Ishii <[email protected]>
Auto-Submit: Kent Tamura <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1440098}

Author: tkent-google

css/css-ruby/ruby-inlinize-fieldset-crash.html

@@ -6,3 +6,8 @@
 </object>
 </i>
 <rt>ô6Ì9ºÍ¶æ</ruby>
+
+<!-- The following is a testcase for a regression by the fix for the above.
+     crbug.com/406999114 -->
+<svg style="display:none" id="ref-parent"><path id="ref"></path></svg>
+<ruby><svg><use xlink:href="#ref"/></svg></ruby>