Refactor client visibility handling

Refactor client visibility handling

Currently, ContextSupport::SetClientVisibile relies on the caller
remembering to pair visible/not-visible calls. Failure to do so will
result in a "leak", where the context support will always think there
are visible clients.

To avoid this, visibility handling now uses ScopedVisibility objects
which will dcheck if visible > not visible updates are not paired.

This change also introduces a new ContextCacheController object which
handles trimming context caches based on visibility. This simplifies
handling in clients of the ContextProvider and removes this logic
from ContextSupport.

CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel

Review-Url: https://codereview.chromium.org/2278283003
Cr-Commit-Position: refs/heads/master@{#415491}
(cherry picked from commit f6c4fb6b0b77f35f094cb73c49b0bfe22d3f4461)

BUG=642836

[ericrk, 2016-09-20 15:56:58]

Committed patchset #1 (id:1) to pending queue manually as
db177352b5ce14d6fe993a01ca335ee56f16ef08.