Added few validations in Direct Write Font Cache code.

Added few validations in Direct Write Font Cache code.
- Added exclusive_write while writing to cache file, so that if there is any
case where two utility processes try to write to cache at same time, it could be prevented.
- Also added check to read minimum structure size bytes.

BUG=434503
R=cpu,ananta

Committed: https://crrev.com/bebcbc64bef44e63cbe88db487fb76ff383334f0
Cr-Commit-Position: refs/heads/master@{#305105}

[Shrikant Kelkar, 2014-11-19 18:26:40]

ping

[ananta, 2014-11-19 18:41:28]

lgtm

[cpu_(ooo_6.6-7.5), 2014-11-20 22:22:20]

lgtm

[Shrikant Kelkar, 2014-11-20 22:23:37]

The CQ bit was checked by shrikant@chromium.org

[commit-bot: I haz the power, 2014-11-20 22:25:36]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/733253005/1

[commit-bot: I haz the power, 2014-11-20 23:08:21]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2014-11-20 23:11:21]

Patchset 1 (id:??) landed as
https://crrev.com/bebcbc64bef44e63cbe88db487fb76ff383334f0
Cr-Commit-Position: refs/heads/master@{#305105}