AW: Fix exception for startActivity from outside of an Activity context

When WebView is initialized by non-activity context, startActivity will
cause runtime exception: lack the FLAG_ACTIVITY_NEW_TASK flag. Now, guard
it with activityFromContext null check.

Bug: 
Change-Id: Ia0ebf88f21d2064b734c0bbc9dc2636457e23321
Reviewed-on: https://chromium-review.googlesource.com/730041Reviewed-by: Bo <boliu@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#512730}

android_webview/java/src/org/chromium/android_webview/AwContentsClient.java

@@ -210,6 +210,12 @@ public abstract class AwContentsClient {
         // same application can be opened in the same tab.
         intent.putExtra(Browser.EXTRA_APPLICATION_ID, context.getPackageName());
 
+        // Check whether the context is activity context.
+        if (AwContents.activityFromContext(context) == null) {
+            Log.w(TAG, "Cannot call startActivity on non-activity context.");
+            return false;
+        }
+
         try {
             context.startActivity(intent);
         } catch (ActivityNotFoundException ex) {