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This patch is a subtask of "Decoupling Gamepad from //content".

In this patch the GamepadService, GamepadServiceUnittest, GamepadMonitor and
GamepadSharedBuffer are moved out of content/browser/gamepad or render_host,
and into device/gamepad/.

The file //content/common/gamepad_hardware_buffer.h is removed, we move the
definition of GamepadHardwareBuffer into gamepad_shared_memory.h, and change
its definition by using generic SharedMemorySeqLockBuffer<blink::WebGamepads>.

In the unittest of GamepadProvider, we don't pass a MockSharedMemoryBuffer
(which has no seqlock controls) anymore. GamepadProvider will create a
SharedMemoryBuffer by default in its constructor, so we add the seqlock-read
accordingly in its unittest.

Based on a patch by bajones@chromium.org.

BUG = 612330

Review-Url: https://codereview.chromium.org/2563483006
Cr-Commit-Position: refs/heads/master@{#437716}