Get the linen background painting during elastic scrolling

We weren't setting the location of the resource bundle correctly in the
renderer process. It turns out base::mac::OuterBundle() returns the helper
app's bundle in the renderer process. So now we have MainApplicationBundle() to
give us the bundle of the main app.

brightray/common/mac/main_application_bundle.h

@@ -0,0 +1,20 @@
+#ifndef BRIGHTRAY_COMMON_MAC_MAIN_APPLICATION_BUNDLE_H_
+#define BRIGHTRAY_COMMON_MAC_MAIN_APPLICATION_BUNDLE_H_
+
+@class NSBundle;
+
+namespace base {
+class FilePath;
+}
+
+namespace brightray {
+
+// The "main" application bundle is the outermost bundle for this logical application. E.g., if you
+// have MyApp.app and MyApp.app/Contents/Frameworks/MyApp Helper.app, the main application bundle is
+// MyApp.app, no matter which executable is currently running.
+NSBundle* MainApplicationBundle();
+base::FilePath MainApplicationBundlePath();
+
+}
+
+#endif