Refactor bindings generation in GYP

A reland of https://codereview.chromium.org/665083008/
Which was reverted https://codereview.chromium.org/694023002/

mojom_list_outputs.py now normalizes the paths it outputs to use forward slashes.
This avoids GYP interpreting backslashes as escape sequences.
Script and template file lists were factored into a shared gypi.
The action now explicitly depends on the input files, the rule has an implicit dependency.
Move generated_src_dirs into the second level of direct_dependent_settings to fix Android.

BUG=https://code.google.com/p/chromium/issues/detail?id=401761
R=jamesr@chromium.org

Review URL: https://codereview.chromium.org/717873004