Fix chromium.content.browser.InterstitialPageTest#testCloseInterstitial on android bots. This was a regression from r242533.

BUG=304341
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/110443011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242543 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242543
Always use the ashmem allocator when creating DiscardableMemory instances.                                                                                                                                                                                                                                                     

Experiments and measurements showed that the possibly accumulated discardable
external fragmentation caused by the allocator's free chunks doesn't matter
much in practice.
The fact that (unpinned) free chunks could be contributing to private dirty was 
the main reason to use the allocator only as a fallback (when file descriptor
usage gets too high). However experimentation showed that unpinned ashmem
eviction happened without degrading user experience (i.e. without killing other
apps nor causing jankiness).

This significantly simplifies the DiscardableMemory creation path on Android
and should provide a speed boost too (10x on Nexus10) in addition to the fact
that ashmem creation + mmap() appeared to have very unpredictable performance.

Review URL: https://codereview.chromium.org/98213004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242544 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242544