Web MIDI: loop style change to be robust

Web MIDI: loop style change to be robust

Potentially, client callbacks invoked inside the loop may touch
the container indirectly. This patch takes such cases just in case.

BUG=469416
TEST=LayoutTests/webmidi

Committed: https://crrev.com/e841a2aeca894ef5b8cc8b0537cd4dd07c266e5e
Cr-Commit-Position: refs/heads/master@{#321982}

[Takashi Toyoshima, 2015-03-24 07:22:44]

toyoshim@chromium.org changed reviewers:
+ yhirano@chromium.org

[Takashi Toyoshima, 2015-03-24 07:39:09]

ptal

[yhirano, 2015-03-24 08:59:17]

lgtm

[Takashi Toyoshima, 2015-03-24 12:52:44]

The CQ bit was checked by toyoshim@chromium.org

[commit-bot: I haz the power, 2015-03-24 12:52:59]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1033603002/1

[commit-bot: I haz the power, 2015-03-24 12:56:54]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-03-24 12:57:55]

Patchset 1 (id:??) landed as
https://crrev.com/e841a2aeca894ef5b8cc8b0537cd4dd07c266e5e
Cr-Commit-Position: refs/heads/master@{#321982}