Add KeepaliveImpulse to extension process manager.

Impulses are an efficient way to handle certain types
of activity from extensions. E.g. the work in progress
to keep NaCl plugins in background pages alive. An
Increment / Decrement approach would require extensive
and fragile instrumentation of all APIs. Impulses
can be sent upon any api activity and act as an
implicit increment with a timed out decrement.

See design doc:
https://docs.google.com/a/chromium.org/document/d/1mI0lS1rfAf-BAGLmWAEcWy37Xq9dOvgfMx8OqeUMXts/edit#

BUG=298339

Review URL: https://codereview.chromium.org/60613004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238261 0039d316-1c4b-4281-b951-d872f2087c98
8 files changed