Fix a failing test TabsTest#HideKeyboard

It is possible that a few android.view.View APIs are called
after WebContents gets destroyed. This CL keeps them from
accessing the destroyed instance by safeguarding it at
ContentView where they pass through.

Alternatively this could be done at embedder level (i.e. parent
view of ContentView), but this approach reduces duplicated code.

And re-enabled the disabled test.

Bug: 821750
Change-Id: Ied55e7d36c589e6e7e324d081204c9a1294da7b2
Reviewed-on: https://chromium-review.googlesource.com/963149Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543249}

chrome/android/javatests/src/org/chromium/chrome/browser/TabsTest.java

@@ -311,7 +311,6 @@ public class TabsTest {
      * Verify that opening a new tab, switching to an existing tab and closing current tab hide
      * keyboard.
      */
-    @DisabledTest(message = "crbug.com/821750")
     @Test
     @LargeTest
     @Restriction(UiRestriction.RESTRICTION_TYPE_TABLET)

components/content_view/java/src/org/chromium/components/content_view/ContentView.java

@@ -85,7 +85,10 @@ public class ContentView
     }
 
     protected WebContentsAccessibility getWebContentsAccessibility() {
-        return WebContentsAccessibility.fromWebContents(mWebContents);
+        // TODO(jinsukkim): Make it possible to check WebContents directly rather than doing it
+        //     via examining the availiability of ContentViewCore;
+        return getContentViewCore() != null ? WebContentsAccessibility.fromWebContents(mWebContents)
+                                            : null;
     }
 
     @Override
@@ -133,12 +136,14 @@ public class ContentView
 
     @Override
     public InputConnection onCreateInputConnection(EditorInfo outAttrs) {
+        if (getContentViewCore() == null) return null;
         return ImeAdapter.fromWebContents(mWebContents).onCreateInputConnection(outAttrs);
     }
 
     @Override
     public boolean onCheckIsTextEditor() {
-        if (mWebContents == null) return false;
+        if (getContentViewCore() == null) return false;
+
         return ImeAdapter.fromWebContents(mWebContents).onCheckIsTextEditor();
     }