Create platform/mojo and move MojoHelper.h there

so that:
- we can avoid having base/bind.h in top-level DEPS in platform
- other mojo-related files, e.g. mojo typemaps to convert things like URLs and origins between mojo types and blink types can be there
- more structure in platform/ in general

BUG=n/a
R=haraken@chromium.org,esprehn@chromium.org,rouslan@chromium.org

Review URL: https://codereview.chromium.org/1910373002

Cr-Commit-Position: refs/heads/master@{#389047}
8 files changed