Add discardable memory mgr in "app" for content browser

"DiscardableSharedMemoryManager" interface is needed by two services:
profiling and pdf_compositor. It was included in "utility" before which
was removed. Now add it in "app" so services can use it.

BUG=455764

Change-Id: I5fe0f8299cd2a927b158c02c13a87db65867a32b
Reviewed-on: https://chromium-review.googlesource.com/643788
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#498930}
3 files changed