The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/f303a2c28ad93a3aeb09469782454b30ac6c9915

commit f303a2c28ad93a3aeb09469782454b30ac6c9915
Author: rockot <rockot@chromium.org>
Date: Thu Feb 23 02:51:57 2017

Mojo C++ Bindings: Introduce build-time option to disable typemapping

Introduces the GN arg |enable_mojom_typemapping| which defaults to
|true|.

Setting this to |false| is strictly for use in non-Chromium build
configurations, e.g. external projects which wish to use Mojo bindings
and which may not wish to mirror the entire Chromium source tree.

When set to |false|, it avoids a build-time dependency on the global
Chromium and Blink typemap configurations.

BUG= 695242 
R=sammc@chromium.org

Review-Url: https://codereview.chromium.org/2715533004
Cr-Commit-Position: refs/heads/master@{#452359}

[modify] https://crrev.com/f303a2c28ad93a3aeb09469782454b30ac6c9915/mojo/public/tools/bindings/mojom.gni

People generally like typemapping quite a bit and it is likely that you may eventually want typemapping, just with your own configuration.

For example, if you ever intend to pass URLs over your mojo interfaces, you would benefit from using the mojom and typemap in //url, which lets such message parameters use the GURL type directly and bakes in GURL-defined validation on deserialization.

I think this can be raised as a separate issue if and when it's deemed necessary. Calling this one fixed.