Change outputting ints in win/linux GetCpuUsage to doubles.

Change outputting ints in win/linux GetCpuUsage to doubles. Note the line I use in win is the same as currently in mac.

I looked into refactoring the methods to use a common strategy but there isn't that much reuse since there are a lot of differences - the only common part is the conversion to percent which is a one-liner, so I left it.

BUG=490879
Committed: https://crrev.com/4fb9a2c47c9412f135d43b00b792171acb26d14c
Cr-Commit-Position: refs/heads/master@{#331369}

[rkaplow, 2015-05-25 15:23:47]

rkaplow@chromium.org changed reviewers:
+ thestig@chromium.org

[Lei Zhang, 2015-05-25 19:41:26]

lgtm

[rkaplow, 2015-05-26 14:23:05]

The CQ bit was checked by rkaplow@chromium.org

[commit-bot: I haz the power, 2015-05-26 14:23:31]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1160533002/1

[commit-bot: I haz the power, 2015-05-26 16:05:08]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-05-26 16:06:35]

Patchset 1 (id:??) landed as
https://crrev.com/4fb9a2c47c9412f135d43b00b792171acb26d14c
Cr-Commit-Position: refs/heads/master@{#331369}