Mojo JS Bindings: Support for complex Map value types

Added JS codec support for a MapOf type and used it to
support complex Mojo map value types.

BUG=
R=yzshen@chromium.org

Committed: https://chromium.googlesource.com/external/mojo/+/e08bb2abe3af7e1dbb00594edf4b7f6efbe3e17e

Review URL: https://codereview.chromium.org/786403002