Fix VS 2015 warning by adding an additional cast

Fix VS 2015 warning by adding an additional cast

The OwningThread member of CRITICAL_SECTION is declared as having type
HANDLE but it is actually the thread's Thread ID which is a DWORD. When
doing 64-bit builds of Chromium with VS 2015 this triggers a warning
because of the suspicious conversion from 32-bit integer to 64-bit
pointer.

This change adds a cast (and some comments) to make the conversion
explicit and avoid the warning.

R=henrikg@webrtc.org
BUG=440500
Committed: https://crrev.com/a10492ff33152cef5a835761125219d3e4ee21d0
Cr-Commit-Position: refs/heads/master@{#10190}

[Henrik Grunell WebRTC, 2015-10-06 19:47:54]

lgtm

[brucedawson, 2015-10-06 19:50:07]

The CQ bit was checked by brucedawson@chromium.org

[commit-bot: I haz the power, 2015-10-06 19:50:17]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1386183002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1386183002/1

[commit-bot: I haz the power, 2015-10-06 20:34:37]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-10-06 20:34:39]

Patchset 1 (id:??) landed as
https://crrev.com/a10492ff33152cef5a835761125219d3e4ee21d0
Cr-Commit-Position: refs/heads/master@{#10190}