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After gamepad mojoification, In renderer side, GamepadSharedMemoryReader has to
"unwrap" the received mojo::ScopedSharedBufferHandle into a
base::SharedMemoryHandle. It is not good.

In this patch we let GamepadSharedMemoryReader completely use mojo::SharedBuffer
to avoid the "unwrap" operation.

BUG = 670655

Review-Url: https://codereview.chromium.org/2567713003
Cr-Commit-Position: refs/heads/master@{#438039}