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Align ashmem region sizes to page size after ashmem creation failed.                                                                                                                                                                                                                                                           

When ashmem creation failed the new size (twice smaller) used to retry wasn't
page-aligned.

This also moves the page alignment function to the allocator's anonymous
namespace since it's now an implementation detail (since the allocator is
always used).

BUG=331832

Review URL: https://codereview.chromium.org/136723005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245201 0039d316-1c4b-4281-b951-d872f2087c98