content/gpu: Create a framework for testing GpuChannel and friends.
GpuChannelManager, GpuChannel, etc. have a lot of untested code around
handling IPC messages. This CL attempts to fix that by making it easy
to add such tests. This was possible by some refactoring:
1. Made test classes for GpuChannelManager and GpuChannel that override
the Send method and other related IPC methods.
2. Removed dependencies on base::ThreadTaskRunnerHandle and
base::MessageLoop::current.
3. Had to work around code which would cause IPC messages to be sent in
a destructor.
BUG=514809
Review URL: https://codereview.chromium.org/
1283973002
Cr-Commit-Position: refs/heads/master@{#344705}