Mojo: Automatically prepend enum names to enum values for C++ bindings.

Mojo: Automatically prepend enum names to enum values for C++ bindings.

With this change the following mojom input
enum Shape {
  CIRCLE,
};

will generate the following C++:
enum Shape {
  SHAPE_CIRCLE,
};

instead of
enum Shape {
  CIRCLE,
};

This also changes expressions of the form Shape.SHAPE_CIRCLE to
Shape.CIRCLE in the Java and JS bindings.

BUG=343981

Review URL: https://codereview.chromium.org/389613007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@283407 0039d316-1c4b-4281-b951-d872f2087c98