Fix MSVC warnings in prtime.cc about potential value truncation by inserting

Fix MSVC warnings in prtime.cc about potential value truncation by inserting
explicit casts.

I didn't check whether there were other, better fixes here by changing more
underlying types/behaviors; I assumed the existing code was correct and just
made the typecasts explicit.

BUG=81439
TEST=none

Committed: https://crrev.com/9883dd4a348cf1e23e4e123b59daf2e2b14a3391
Cr-Commit-Position: refs/heads/master@{#296668}

[Peter Kasting, 2014-09-23 23:52:14]

pkasting@chromium.org changed reviewers:
+ rsleevi@chromium.org

[Ryan Sleevi, 2014-09-23 23:58:13]

lgtm

[Peter Kasting, 2014-09-25 06:02:11]

The CQ bit was checked by pkasting@chromium.org

[commit-bot: I haz the power, 2014-09-25 06:03:01]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/601593002/1

[commit-bot: I haz the power, 2014-09-25 06:34:26]

Committed patchset #1 (id:1) as 832d1851d5d8b40b0937ae15d48743ecc585b79d

[commit-bot: I haz the power, 2014-09-25 06:35:26]

Patchset 1 (id:??) landed as
https://crrev.com/9883dd4a348cf1e23e4e123b59daf2e2b14a3391
Cr-Commit-Position: refs/heads/master@{#296668}