Re-add removed update to theme color in web_contents_impl

The update to the theme color was removed here:
https://chromium-review.googlesource.com/c/chromium/src/+/2343269

Bug: 1121108
Change-Id: I6af81f699c69b4c919fed02c6777a4765e1b5485
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2372805
Auto-Submit: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801357}

content/browser/web_contents/web_contents_impl.cc

@@ -5093,6 +5093,7 @@ void WebContentsImpl::OnThemeColorChanged(RenderViewHostImpl* source) {
     observers_.ForEachObserver([&](WebContentsObserver* observer) {
       observer->DidChangeThemeColor();
     });
+    last_sent_theme_color_ = source->theme_color();
   }
 }