Disk cache: use mmap for cache index on x86.
Disk cache: use mmap for cache index on x86.

We are pretty confident that mmap-ing the index would not hurt any existing x86
android devices, but we cannot be so sure about the variety of ARM devices. So
enable it for x86 only for now.

BUG=170344
TEST=instrument mapped_file_avoid_mmap_posix.cc:Flush() and make sure it is not
     invoked when run on an x86 device


Review URL: https://chromiumcodereview.appspot.com/12317012

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@183610 0039d316-1c4b-4281-b951-d872f2087c98

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