blink/bindings: Handle foreground OnPurgeMemory() signals.

OnPurgeMemory() happens in case of memory pressure, and also when a renderer has
been backgrounded for some time. If received while in foreground, then we should
reduce memory footprint as much as possible.

To that end:
- Park all parkable strings
- Drop all compressed representation from strings that cannot be parked.

Bug: 877044
Change-Id: I6119fbf64f4954a9abe3b8466f7e7cbdff807e8f
Reviewed-on: https://chromium-review.googlesource.com/c/1348061
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613509}
5 files changed