aw: Check if destroyed before using mWebContents in onDragEvent

Bug: 789168
Change-Id: Ib90d86b1e286c3e5c5e2b8d13adde4bf7edc9b34
Reviewed-on: https://chromium-review.googlesource.com/793818Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Tobias Sargeant <tobiasjs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519918}

android_webview/java/src/org/chromium/android_webview/AwContents.java

@@ -3282,7 +3282,9 @@ public class AwContents implements SmartClipProvider {
 
         @Override
         public boolean onDragEvent(DragEvent event) {
-            return mWebContents.getEventForwarder().onDragEvent(event, mContainerView);
+            return isDestroyedOrNoOperation(NO_WARN)
+                    ? false
+                    : mWebContents.getEventForwarder().onDragEvent(event, mContainerView);
         }
 
         @Override