[TTS] Fix tap recognition in Reader Mode.

Fixes the recent regression with tap gesture recognition in Reader
mode tabs.  Converting a tab to show the simplified view is one of
the rare times when we get a new CVC or WebContents for the same tab.
With this change we check if the tab is connected to a different
WebContents and create a new observer when it is.

BUG=821676

Change-Id: I8ba56ffba42a6ca9c138c2cd1c387e398cc27591
Reviewed-on: https://chromium-review.googlesource.com/962992Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543255}

chrome/browser/android/contextualsearch/contextual_search_tab_helper.cc

@@ -59,7 +59,9 @@ void ContextualSearchTabHelper::InstallUnhandledTapNotifierIfNeeded(
   content::WebContents* base_web_contents =
       content::WebContents::FromJavaWebContents(j_base_web_contents);
   DCHECK(base_web_contents);
-  if (!unhandled_tap_web_contents_observer_) {
+  if (!unhandled_tap_web_contents_observer_ ||
+      base_web_contents !=
+          unhandled_tap_web_contents_observer_->web_contents()) {
     unhandled_tap_web_contents_observer_.reset(
         new contextual_search::UnhandledTapWebContentsObserver(
             base_web_contents, device_scale_factor,