Add discardable memory emulation for non-android/mac platforms
Adds support for emulated discardable memory. The memory is managed by a
provider which listens for memory pressure notifications from the platform.
Currently, only android pushes these notifications, but in future patches, we
will apply pressure on other platforms in certain situations (e.g., when a tab
gets backgrounded).
BUG=237681
Review URL: https://codereview.chromium.org/
17106004
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231845 0039d316-1c4b-4281-b951-d872f2087c98