Fix crash in ContentViewCore.onRotationChanged

Reporte crash can happen if |ContentViewCore.onRotationChanged| is called
after CVC is destroyed, therefore mWebContents is set to null. Added
null check against the variable to prevent crash.

Bug: 808971
Change-Id: I7719d2d0c26b36b94eeff406d655512111e58c2d
Reviewed-on: https://chromium-review.googlesource.com/903342Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#534835}

content/public/android/java/src/org/chromium/content/browser/ContentViewCoreImpl.java

@@ -1126,9 +1126,11 @@ public class ContentViewCoreImpl
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M
                 && getSelectionPopupController().isActionModeValid()) {
             hidePopupsAndPreserveSelection();
-            getSelectionPopupController().showActionModeOrClearOnFailure();
+            if (mWebContents != null) {
+                getSelectionPopupController().showActionModeOrClearOnFailure();
+            }
         }
-        getTextSuggestionHost().hidePopups();
+        if (mWebContents != null) getTextSuggestionHost().hidePopups();
 
         int rotationDegrees = 0;
         switch (rotation) {