Plugin service: Use the correct registry key for each watch operation.

Plugin service: Use the correct registry key for each watch operation.

6f315b85b9126cdbbeee5 master@{#300036} introduced a regression where the two
registry keys that are watched for changes to detect installation of NPAPI
plugins, were being mixed.

This CL fixes the issue... using the correct key each time a new StartWatching
is called.

BUG=428560
R=piman@chromium.org

Committed: https://crrev.com/b805615793e5ddcf7aae67f90672027d76857f29
Cr-Commit-Position: refs/heads/master@{#303347}

[rvargas (doing something else), 2014-11-07 23:41:47]

rvargas@chromium.org changed reviewers:
+ piman@chromium.org
- sievers@chromium.org

[rvargas (doing something else), 2014-11-07 23:41:59]

PTAL

[piman, 2014-11-07 23:56:32]

lgtm

[rvargas (doing something else), 2014-11-08 00:38:34]

The CQ bit was checked by rvargas@chromium.org

[commit-bot: I haz the power, 2014-11-08 00:39:58]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/708933002/1

[commit-bot: I haz the power, 2014-11-08 01:44:35]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-08 01:45:11]

Patchset 1 (id:??) landed as
https://crrev.com/b805615793e5ddcf7aae67f90672027d76857f29
Cr-Commit-Position: refs/heads/master@{#303347}