Fix bug in DiscardableMemoryAllocatorAndroid.

AshmemRegion keeps track of the start address of the highest allocated chunk in
the region.
In case the highest allocated chunk is freed then gets reused and split because
it's too large, |AshmemRegion::highest_allocated_chunk_| needs to be updated as
well. This was leading to invalid chaining of previous chunks.

BUG=347919
R=mark@chromium.org

Review URL: https://codereview.chromium.org/184103015

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@254755 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 254755