This patch ensures that all code path of the external memory pressure
handler pass the kGCCallbackFlagCollectAllExternalMemory flag to the
embedder. This is necessary to force Oilpan GC.

Bug: chromium:733655
Change-Id: I2e680d1e7f9087d1930d56ead39ef7cf1bc93e56
Reviewed-on: https://chromium-review.googlesource.com/616149Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47378}