Using DidCommitProvisionalLoad() instead of DidStartProvisionalLoad() in USIS.

Using DidCommitProvisionalLoad() instead of DidStartProvisionalLoad() in USIS.

When the download is started, DidStartProvisionalLoad() is called which
in-turn causes UserScriptIdleSchedule to abandon its user scrips. However,
the provisional load is then abandoned. To fix this, this patch only informs
the USIS of a provisional load on DidCommitProvisionalLoad(), which does not
happen on a downlaod attempt.

BUG=104853
TEST=Have an extension that calls chrome.tabs.executeScript().
Start a download that does not redirect the page. Now try having the
extension executeScript().

Before this patch: the inject will not happen.
After this patch: the inject will happen.

[Matt Perry, 2012-03-30 20:43:21]

lgtm

[commit-bot: I haz the power, 2012-03-30 20:51:04]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/eaugusti@chromium.org/9844008/1

[commit-bot: I haz the power, 2012-03-30 22:06:42]

Try job failure for 9844008-1 (retry) on linux_rel for step "browser_tests".
It's a second try, previously, step "browser_tests" failed.
http://build.chromium.org/p/tryserver.chromium/buildstatus?builder=linux_rel&...