Revert of Revert of Make sure MessagePortService is used in a thread safe manner.

Revert of Revert of Make sure MessagePortService is used in a thread safe manner. (patchset #1 id:1 of https://codereview.chromium.org/942583005/)

Reason for revert:
This does not seem to cause the problem.

Original issue's description:
> Revert of Make sure MessagePortService is used in a thread safe manner. (patchset #4 id:80001 of https://codereview.chromium.org/937503003/)
>
> Reason for revert:
> This CL is suspected to cause the Win7 test (dbg) failures.
>
> Original issue's description:
> > Added DCHECKs to all public methods of MessagePortService to make sure it is used in a thread safe manner.
> >
> > BUG=459115
> >
> > Committed: https://crrev.com/c2d9552dde5a7dd05dc63e744c71f497f0120306
> > Cr-Commit-Position: refs/heads/master@{#317178}
>
> TBR=creis@chromium.org,sgurun@chromium.org
> NOPRESUBMIT=true
> NOTREECHECKS=true
> NOTRY=true
> BUG=459115
>
> Committed: https://crrev.com/60583adc54a6b043dc0b812f10869801eb0da0b4
> Cr-Commit-Position: refs/heads/master@{#317299}

TBR=creis@chromium.org,sgurun@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=459115
Committed: https://crrev.com/721247d2d4fb18b27c0ccd018c96011b74602a2e
Cr-Commit-Position: refs/heads/master@{#317312}

[kaliamoorthi, 2015-02-20 14:07:26]

Created Revert of Revert of Make sure MessagePortService is used in a thread
safe manner.

[commit-bot: I haz the power, 2015-02-20 14:08:09]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/947483002/1

[commit-bot: I haz the power, 2015-02-20 14:09:18]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-02-20 14:10:08]

Patchset 1 (id:??) landed as
https://crrev.com/721247d2d4fb18b27c0ccd018c96011b74602a2e
Cr-Commit-Position: refs/heads/master@{#317312}