Add a feature flag to purge the WebCache on MemoryPressure only

It's not clear if the logic that tries to divide a total WebCache budget
between all renderers is still needed or if we should just trim the
caches a few minutes after backgrounding a renderer, the goal of this
feature is to answer this question.

The current logic generates many IPC and wakeups in background
renderers.

Bug: 1199908
Change-Id: I886028e5649a6522249d0fe0b36628c08e6556f6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2838245
Commit-Queue: Sébastien Marchand <sebmarchand@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#874968}