Add zoomBy API plumbing

This adds the plumbing necessary for the zoomBy API so that it's
possible to land the frameworks/base change that actually adds the
new API to the WebView.

BUG: 13399510
Change-Id: I15924235e6deb25eff68638f174b6cd158b3d902
diff --git a/chromium/java/com/android/webview/chromium/WebViewChromium.java b/chromium/java/com/android/webview/chromium/WebViewChromium.java
index c3f572b..bb0f07e 100644
--- a/chromium/java/com/android/webview/chromium/WebViewChromium.java
+++ b/chromium/java/com/android/webview/chromium/WebViewChromium.java
@@ -1413,6 +1413,17 @@
         return mAwContents.zoomOut();
     }
 
+    //TODO: remove once frameworks/base is updated
+    //@Override
+    public boolean zoomBy(float factor) {
+        mFactory.startYourEngines(true);
+        // This is an L API and therefore we can enforce stricter threading constraints.
+        checkThread();
+        //TODO: remove once Chromium-side CL is merged.
+        //return mAwContents.zoomBy(factor);
+        return false;
+    }
+
     @Override
     public void dumpViewHierarchyWithProperties(BufferedWriter out, int level) {
         // Intentional no-op