Reland: Add MemoryCoordinatorClient

Make MemoryCoordinator garbage collected to make sure that MemoryCoordinator::trace()
is called. Garbage collected MemoryCoordinatorClient will be removed from |m_clients|.

Original description:

Also move MemoryCoordinator from core/dom to platform/. This is a preparation for experimenting purge and suspend [1,2].

[1] https://docs.google.com/document/d/1EgLimgxWK5DGhptnNVbEGSvVn6Q609ZJaBkLjEPRJvI/edit
[2] https://codereview.chromium.org/2130683002/

[email protected],[email protected]
BUG=638274

Review-Url: https://codereview.chromium.org/2251853005
Cr-Commit-Position: refs/heads/master@{#412966}
13 files changed