[webcrypto] Allow crypto operations to be cancelled by the platform implementation.

[webcrypto] Allow crypto operations to be cancelled by the platform implementation.

When the ExecutionContext is destroyed, the outstanding request is marked as cancelled.

The corresponding chromium side of this change is https://codereview.chromium.org/341923004/

BUG=375430
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176502

[abarth-chromium, 2014-06-19 02:07:55]

lgtm

[commit-bot: I haz the power, 2014-06-19 02:08:15]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/eroman@chromium.org/343723003/1

[commit-bot: I haz the power, 2014-06-19 03:20:47]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-06-19 03:20:48]

Try jobs failed on following builders:
  mac_blink_compile_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/mac_blink_compile_dbg/bu...)

[eroman, 2014-06-19 06:59:25]

The CQ bit was checked by eroman@chromium.org

[commit-bot: I haz the power, 2014-06-19 07:00:35]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/eroman@chromium.org/343723003/20001

[eroman, 2014-06-19 07:00:57]

The CQ bit was unchecked by eroman@chromium.org

[eroman, 2014-06-19 07:06:48]

The CQ bit was checked by eroman@chromium.org

[commit-bot: I haz the power, 2014-06-19 07:07:35]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/eroman@chromium.org/343723003/40001

[commit-bot: I haz the power, 2014-06-19 11:57:35]

Change committed as 176502