Enable devtools debugging for WebPlatformTestsActivity.java.

Bug: 1000798
Change-Id: If100bbcd2ed09b18b24f473927d28db94ddb797f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1785088Reviewed-by: Changwan Ryu <changwan@chromium.org>
Commit-Queue: Andrew Luo <aluo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#693714}

android_webview/tools/system_webview_shell/apk/src/org/chromium/webview_shell/WebPlatformTestsActivity.java

@@ -105,6 +105,7 @@ public class WebPlatformTestsActivity extends Activity {
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
+        WebView.setWebContentsDebuggingEnabled(true);
         setContentView(R.layout.activity_web_platform_tests);
         setUpWidgets();
         // This is equivalent to Chrome's WPT setup.