Cleanup GuestViewMessageFilter

Cleanup GuestViewMessageFilter

1. Use auto to improve readability.
2. Rename methods to remove "Extension" prefix.
3. Use const where possible.
4. Use using content::* to improve readability.

BUG=none

Committed: https://crrev.com/90ba99132307524fb5ec891d11454e5f0e604847
Cr-Commit-Position: refs/heads/master@{#313608}

[Fady Samuel, 2015-01-28 18:57:48]

fsamuel@chromium.org changed reviewers:
+ lazyboy@chromium.org

[lazyboy, 2015-01-28 21:07:20]

lgtm

[Fady Samuel, 2015-01-28 21:42:52]

The CQ bit was checked by fsamuel@chromium.org

[commit-bot: I haz the power, 2015-01-28 21:43:42]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/880343002/1

[commit-bot: I haz the power, 2015-01-28 23:15:15]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-01-28 23:16:49]

Patchset 1 (id:??) landed as
https://crrev.com/90ba99132307524fb5ec891d11454e5f0e604847
Cr-Commit-Position: refs/heads/master@{#313608}