Update theme color on content change

This patch updates the theme color when the tab's content changes as
the typical onThemeColorChanged event is not sent.

Bug: 1115010
Change-Id: I26bad29edff317cc71d3c67437727dc6327cbd9d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346978Reviewed-by: Mei Liang <meiliang@chromium.org>
Commit-Queue: Matthew Jones <mdjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797222}

chrome/android/java/src/org/chromium/chrome/browser/compositor/overlays/toolbar/TopToolbarOverlayMediator.java

@@ -111,6 +111,7 @@ public class TopToolbarOverlayMediator {
             @Override
             public void onContentChanged(Tab tab) {
                 updateVisibility();
+                updateThemeColor(tab);
             }
         };