[WebLayer] Fix up exposing of Context via BrowserImpl

Need to check internally for |mWindowAndroid| being null before
accessing it.

Bug: 1025617, 1031465
Change-Id: I7f232dd35a23cc5f82b17d4902b3e52e1e1c2040
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1991443Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#729754}

weblayer/browser/java/org/chromium/weblayer_private/BrowserImpl.java

@@ -109,6 +109,10 @@ public class BrowserImpl extends IBrowser.Stub {
     }
 
     public Context getContext() {
+        if (mWindowAndroid == null) {
+            return null;
+        }
+
         return mWindowAndroid.getContext().get();
     }