Docserver: Rename CompiledFileSystem's GetFileVersion/GetFileListingVersion to

Docserver: Rename CompiledFileSystem's GetFileVersion/GetFileListingVersion to
_GetFileVersionFromCache/_GetFileListingVersionFromCache, because they're
protected not public methods. More importantly, make them return Futures.
Plus a little more cleanup.

These are small changes in preparation to the more interesting patch which
addresses bug 402903.

BUG=402903
TBR=yoz@chromium.org

Committed: https://crrev.com/b147f5ec9548bb32b3a63dae8e063c9c65907eab
Cr-Commit-Position: refs/heads/master@{#292797}

[not at google - send to devlin, 2014-08-30 16:16:24]

kalman@chromium.org changed reviewers:
+ ahernandez.miralles@gmail.com, rockot@chromium.org

[not at google - send to devlin, 2014-08-30 16:16:25]

Hey Yoyo, Ken, Freddy -

Sorry to be TBR'ing this but as you know I'll be OOO for a long time and want to
get a fix in for bug 402903.

If you could do me a huge, huge favour - post-commit review this and either file
a bug against me to fix it (hah) or if it's tiny... maybe... fix it... :)

[not at google - send to devlin, 2014-08-30 16:16:29]

The CQ bit was checked by kalman@chromium.org

[commit-bot: I haz the power, 2014-08-30 16:17:23]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/kalman@chromium.org/522183002/20001

[commit-bot: I haz the power, 2014-08-30 17:14:25]

Committed patchset #2 (id:20001) as 4a2d86eebdcd91db7592fa8a81ac1f6bf4d61073

[commit-bot: I haz the power, 2014-09-10 03:14:30]

Patchset 2 (id:??) landed as
https://crrev.com/b147f5ec9548bb32b3a63dae8e063c9c65907eab
Cr-Commit-Position: refs/heads/master@{#292797}