Fix some more problems with windows paths in content verification

Fix some more problems with windows paths in content verification

It turns out that the relative path we generate inside the network stack
when requesting chrome://extension urls uses '/' path separators for
relative paths, but the in-memory representation of computed_hashes.json
and verified_contents.json files ended up with '\' path separators on
windows. So we ended up not finding any paths with a subdirectory at
request time.

I've fixed this by just standardizing on '/' path separators everywhere.

BUG=382638
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=280473

[Ken Rockot(use gerrit already), 2014-06-27 21:21:28]

lgtm

[asargent_no_longer_on_chrome, 2014-06-27 21:41:16]

The CQ bit was checked by asargent@chromium.org

[commit-bot: I haz the power, 2014-06-27 21:44:53]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/asargent@chromium.org/361523002/1

[commit-bot: I haz the power, 2014-06-28 01:08:28]

Change committed as 280473