This ensures that the hermetic toolchain is only used if use_system_xcode is
false. This CL also causes two changes:
  * svn is assumed to be installed on the system. It is not pulled from the
  hermetic toolchain.
  * mac_sdk_build was used to populate the SDK version in the Info.plist. This
  was being populated with a different version than the SDK being used to build
  Chrome, which is incorrect.

BUG=651267

Review-Url: https://codereview.chromium.org/2412353003
Cr-Commit-Position: refs/heads/master@{#425170}