Bring back lost gamepad_seqlock_unittest.

Bring back lost gamepad_seqlock_unittest.

Currently gamepad_seqlock_unittest.cc is not included in content_tests.gypi.
Also GamepadSeqLock was renamed to OneWriterSeqlLock some time ago as it
is used not only by the gamepad.
To rectify this situation this patch does the following:
- replace references to GamepadSeqLock
- include the unittest in content_tests.gypi

Committed: https://crrev.com/553f87c99cb34e0e95c5a9bf740537ed74163a2e
Cr-Commit-Position: refs/heads/master@{#292878}

[timvolodine, 2014-08-26 13:52:13]

timvolodine@chromium.org changed reviewers:
+ scottmg@chromium.org

[scottmg, 2014-08-26 17:02:18]

scottmg@chromium.org changed reviewers:
+ bajones@chromium.org

[scottmg, 2014-08-26 17:02:18]

lgtm, thanks. +bajones fyi

[timvolodine, 2014-08-29 17:30:57]

timvolodine@chromium.org changed reviewers:
+ jam@chromium.org

[timvolodine, 2014-08-29 17:30:58]

+ jam: as content/common owner

[timvolodine, 2014-09-01 11:52:59]

timvolodine@chromium.org changed reviewers:
+ jochen@chromium.org

[timvolodine, 2014-09-01 11:52:59]

+ jochen for content/common

[jochen (gone - plz use gerrit), 2014-09-01 13:17:22]

lgtm

[timvolodine, 2014-09-01 15:02:36]

The CQ bit was checked by timvolodine@chromium.org

[commit-bot: I haz the power, 2014-09-01 15:03:05]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/timvolodine@chromium.org/507643002/1

[commit-bot: I haz the power, 2014-09-01 16:00:04]

Committed patchset #1 (id:1) as 18687fc9d1fd73bb9d7dd1b5839536e276d32d02

[commit-bot: I haz the power, 2014-09-10 03:17:03]

Patchset 1 (id:??) landed as
https://crrev.com/553f87c99cb34e0e95c5a9bf740537ed74163a2e
Cr-Commit-Position: refs/heads/master@{#292878}