gpu: Assert that FencedAllocator is empty in destructor

With the commit "gpu: Reuse transfer buffers more aggresively" mapped
memory is free:ed on lost context. This means that we can reinstate the
DCHECKs in the FencedAllocator destructor that checks that all blocks
have been free:ed by their owner.

Tests that didn't explicitly free mapped memory now do so, and one test
that tested that leaking was OK has been removed.

BUG=328808

Review URL: https://codereview.chromium.org/282513003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@270991 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed