Reland "Use document background color as active tab color for tabbed web apps"

This is a reland of c4de5e43b76400725469e2ed0b14e0ebce333524

Fixed test flake due to not waiting for the first
OnBackgroundColorChanged() event during page load.
https://chromium-review.googlesource.com/c/chromium/src/+/2367584/2..3/chrome/browser/ui/views/web_apps/web_app_tab_strip_browsertest.cc

Original change's description:
> Use document background color as active tab color for tabbed web apps
>
> This CL hooks up the document's background color as the active tab
> background color for experimental tabbed web app.
> Tabbed web apps are hidden behind the DesktopPWAsTabStrip feature flag.
>
> This CL plumbs background color through from the renderer process along
> the same route as document theme color.
>
> Before: https://bugs.chromium.org/p/chromium/issues/attachment?aid=460591&signed_aid=NcgKQaq5Y47QkcbQbbgNiw==&inline=1
> After: https://bugs.chromium.org/p/chromium/issues/attachment?aid=460590&signed_aid=RlnBzb7mQq1SIW1fGBY-oA==&inline=1
>
> Bug: 1104083
> Change-Id: If1c3e2172f80375a1a4619c17ee52594ef813005
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2342426
> Commit-Queue: Alan Cutter <[email protected]>
> Reviewed-by: Avi Drissman <[email protected]>
> Reviewed-by: Dominick Ng <[email protected]>
> Reviewed-by: Kentaro Hara <[email protected]>
> Reviewed-by: Michael Wasserman <[email protected]>
> Auto-Submit: Alan Cutter <[email protected]>
> Cr-Commit-Position: refs/heads/master@{#800051}

Bug: 1104083
Change-Id: If4a1b525af77ccd4697f6931b59a759a8bf3e3ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2367584
Commit-Queue: Dominick Ng <[email protected]>
Auto-Submit: Alan Cutter <[email protected]>
Reviewed-by: Dominick Ng <[email protected]>
Reviewed-by: Kentaro Hara <[email protected]>
Cr-Commit-Position: refs/heads/master@{#800486}
diff --git a/content/public/browser/web_contents.h b/content/public/browser/web_contents.h
index f1c25ed..bf5155b 100644
--- a/content/public/browser/web_contents.h
+++ b/content/public/browser/web_contents.h
@@ -375,6 +375,10 @@
   // theme-color meta tag if any.
   virtual base::Optional<SkColor> GetThemeColor() = 0;
 
+  // Returns the background color for the underlying content as set by CSS if
+  // any.
+  virtual base::Optional<SkColor> GetBackgroundColor() = 0;
+
   // Returns the committed WebUI if one exists, otherwise the pending one.
   virtual WebUI* GetWebUI() = 0;
   virtual WebUI* GetCommittedWebUI() = 0;