Use different file sizes when testing FilesystemCopierAction.
Use different file sizes when testing FilesystemCopierAction.

* In attempt to nail down the reason for failure in unit test
  FilesystemCopierActionTest.RunAsRootSimpleTest, we're trying to
  initialize the input/output files to different sizes.

* In doing so, we're extending/fixing the semantics of the reusable
  DoTest() method: (a) it now returns a truth value that corresponds to
  success/failure, in addition to failing the test internally, which
  allows us to assert the success of a particular invocation of
  DoTest();  (b) it now correctly interrupt the test on critical errors,
  such as failing to write/read files, etc.

BUG=chromium-os:29841
TEST=Unit tests run successfully

Change-Id: Ie6a2aa981e3c86e545f3dafcc66129a5f47bb705
Reviewed-on: https://gerrit.chromium.org/gerrit/22943
Reviewed-by: Darin Petkov <78a9f901370f79bffab0f0650001fa5e6b8f6fd2@chromium.org>
Commit-Ready: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>
Tested-by: Gilad Arnold <a0dab390cfdc583218cee97b60c5c4eb1be355d2@chromium.org>

mjg59/update_engine,marineam/update_engine,marineam/update_engine,endocode/update_engine,endocode/update_engine,marineam/update_engine,endocode/update_engine,marineam/update_engine,mjg59/update_engine,mjg59/update_engine