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This CL organizes the typemaps in Blink GN build. Previously, we dump
typemaps in Blink to third_party/WebKit/Source/platform/mojo/blink_typemaps.gni,
even though some of them are used by third_party/WebKit/public/. This is wrong
and inconsistent with GYP build. So I added a new typemaps.gni in WebKit/public
to collect them separately.

TBR=pdr@chromium.org

BUG=629566

Review-Url: https://codereview.chromium.org/2172333002
Cr-Commit-Position: refs/heads/master@{#407277}