[Device Service] Move Gamepad Blink headers to be part of the Gamepad client library

This CL follows the approach of
https://codereview.chromium.org/2415083002
and moves
//third_party/WebKit/public/platform/WebGamepad.h
//third_party/WebKit/public/platform/WebGamepads.h
to
//device/gamepad/public/cpp/
and puts them in a special target to indicate that they're shared with
Blink

BUG=695012

Review-Url: https://codereview.chromium.org/2808093006
Cr-Commit-Position: refs/heads/master@{#465129}
96 files changed