content: Improve HostDiscardableSharedMemoryManager's memory pressure handling.

content: Improve HostDiscardableSharedMemoryManager's memory pressure handling.

This reduces the chance of an OOM error by handling moderate
memory pressure signals. HostDiscardableSharedMemoryManager will
purge discardable memory until usage is within half of |memory_limit_|
when receiving this signal.

BUG=460995
Committed: https://crrev.com/76741807d75d891fdadf1a7d4635be7ee35c13f7
Cr-Commit-Position: refs/heads/master@{#318108}

[reveman, 2015-02-25 19:44:23]

reveman@chromium.org changed reviewers:
+ avi@chromium.org

[Avi (use Gerrit), 2015-02-25 20:29:47]

lgtm

Sure!

[reveman, 2015-02-25 20:30:32]

The CQ bit was checked by reveman@chromium.org

[commit-bot: I haz the power, 2015-02-25 20:31:07]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/944823007/1

[commit-bot: I haz the power, 2015-02-25 20:48:05]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-25 20:48:41]

Patchset 1 (id:??) landed as
https://crrev.com/76741807d75d891fdadf1a7d4635be7ee35c13f7
Cr-Commit-Position: refs/heads/master@{#318108}