This time for the current memory size. This call also used to use the
context object stored in the instance, hence it required the instance
to be set. This is no longer the case, so the DCHECKs can just be
removed.

R=ahaas@chromium.org
BUG=chromium:727222

Change-Id: I72a7e3e80c3beb15ecad00c5be068e803456797e
Reviewed-on: https://chromium-review.googlesource.com/517947Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45587}