Ignore double cancels of serial reads and writes.
Ignore double cancels of serial reads and writes.

Previously, two CancelRead calls on the same message loop tick would
cause QueueReadCompleted to be called twice, triggering a second call to
ReadCompleted with pending_read_buffer_ cleared. With this change, extra
calls to CancelRead and CancelWrite have no effect.

BUG=401252

Review URL: https://codereview.chromium.org/461533002

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@288653 0039d316-1c4b-4281-b951-d872f2087c98

littlstar/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,littlstar/chromium.src,littlstar/chromium.src,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,bright-sparks/chromium-spacewalk,littlstar/chromium.src,bright-sparks/chromium-spacewalk,bright-sparks/chromium-spacewalk,ondra-novak/chromium.src,littlstar/chromium.src,littlstar/chromium.src,littlstar/chromium.src