Fix InMemoryUrlProtocol available_bytes computation.

      Fixed available_bytes computation to properly handle large buffers and
add defensive logic to Read() to protect against negative sizes.

BUG=364065
TESTS=InMemoryUrlProtocolTest.*

Review URL: https://codereview.chromium.org/253923002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@267280 0039d316-1c4b-4281-b951-d872f2087c98