Use DownloadItem::GetState() in c/b/{download,extensions}

Stop using DownloadItem::Is*() functions that have an equivalent
state exposed via GetState(). Rationale is that there shouldn't be
multiple ways of getting the same state information.

To make review easier, this patch covers chrome/browser/download
and chrome/browser/extensions. The remaining cases and changing
the DownloadItem to remove the Is*() functions will come later.

TEST=browser_tests and unit_tests both with *Download*:*SavePage* filters

Review URL:

git-svn-id: svn:// 0039d316-1c4b-4281-b951-d872f2087c98
20 files changed