Add flag to WebView apks specifying that these apks contain WebView.

Add a flag to the WebView manifest to ensure that webview apks are
compatible with the new mechanism for loading webview.

BUG=

Review URL: https://codereview.chromium.org/1075883002

Cr-Commit-Position: refs/heads/master@{#324418}

android_webview/apk/java/AndroidManifest.xml

@@ -30,5 +30,7 @@
         <provider android:name="com.android.webview.chromium.LicenseContentProvider"
                   android:exported="true"
                   android:authorities="com.android.webview.chromium.LicenseContentProvider" />
+        <meta-data android:name="com.android.webview.WebViewLibrary"
+                   android:value="libwebviewchromium.so" />
     </application>
 </manifest>