Android: Trigger tab placeholder update when activity is paused

This makes sure we have a placeholder bitmap when we resume
and don't have a frame from the renderer yet.

BUG=636630

Review-Url: https://codereview.chromium.org/2186453004
Cr-Commit-Position: refs/heads/master@{#414822}

chrome/android/java/src/org/chromium/chrome/browser/ChromeActivity.java

@@ -671,6 +671,10 @@ public abstract class ChromeActivity extends AsyncInitializationActivity
     @Override
     public void onPauseWithNative() {
         RecordUserAction.record("MobileGoToBackground");
+        Tab tab = getActivityTab();
+        if (tab != null) {
+            getTabContentManager().cacheTabThumbnail(tab);
+        }
         markSessionEnd();
         super.onPauseWithNative();
     }