Don't update the title to about:blank on first show for custom tabs

Currently custom tabs don't show a title if the title is still the url,
but due to optimizations inside warmup(), the title might end up being
about:blank (even if the current url is not about:blank). The domain
explicitly guards against this but title doesn't. Update the title to
also never show about:blank

BUG=579647

Review URL: https://codereview.chromium.org/1649183002 .

Cr-Commit-Position: refs/branch-heads/2623@{#201}
Cr-Branched-From: 92d7753-refs/heads/master@{#369907}

chrome/android/java/src/org/chromium/chrome/browser/toolbar/CustomTabToolbar.java

@@ -214,7 +214,10 @@ public void setTitleToPageTitle() {
 
         // It takes some time to parse the title of the webcontent, and before that Tab#getTitle
         // always return the url. We postpone the title animation until the title is authentic.
-        if (mShouldShowTitle && !TextUtils.equals(currentTab.getTitle(), currentTab.getUrl())) {
+        // TODO(yusufo): Clear the explicit references to about:blank here and for domain.
+        if (mShouldShowTitle
+                && !TextUtils.equals(currentTab.getTitle(), currentTab.getUrl())
+                && !TextUtils.equals(currentTab.getTitle(), "about:blank")) {
             long duration = System.currentTimeMillis() - mInitializeTimeStamp;
             if (duration >= TITLE_ANIM_DELAY_MS) {
                 mTitleAnimationStarter.run();