aw: Try to turn crash into java exception

Since I can't work out why things are null in native code anymore.

BUG=618807

Review-Url: https://codereview.chromium.org/2076633002
Cr-Commit-Position: refs/heads/master@{#400690}

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

@@ -1578,6 +1578,10 @@ public class AwContents implements SmartClipProvider,
             }
         }
 
+        // Temporary to generate a Java stack for crbug.com/618807.
+        if (mNativeAwContents == 0) {
+            throw new RuntimeException("Calling load on destroyed webview " + mIsDestroyed);
+        }
         nativeSetExtraHeadersForUrl(
                 mNativeAwContents, params.getUrl(), params.getExtraHttpRequestHeadersString());
         params.setExtraHeaders(new HashMap<String, String>());