base: Add uncached malloc based discardable memory type.

Each DiscardableMemoryMalloc instance uses a separate malloc() allocated block
of memory. It's allocated on Lock() and freed on Unlock().

This is useful to test how discardable memory clients perform in situations
when the memory pressure is high. The base class will also be used by other
discardable memory implementations as a fall-back when failing to
allocate/lock native discardable memory.

BUG=334996
TEST=base_unittests --gtest_filter=DiscardableMemoryTests*

Review URL: https://codereview.chromium.org/145643008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250167 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250167