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This is to better handle low memory situations when applying a differential
patch to a large Chrome setup.

For reading input files, I'm also switching to using memory mapped files
instead of ReadFileToString to reduce the load on the heap.

TEST=courgette.exe should succeed in applying a patch between two
  chrome 10.x archives on an XP machine with 180MB of physical memory and
  no page file.
BUG=72459,73209

Review URL: http://codereview.chromium.org/6597038

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@76320 0039d316-1c4b-4281-b951-d872f2087c98