On browser close, the browser checks to see if there are any pending downloads.

Currently: If there are any pending downloads, it halts the download and asks the user to keep or save the download.

New behavior: The user won't be prompted for pending downloads that are malicious (DANGEROUS_HOST, DANGEROUS_URL, DANGEROUS_CONTENT). Instead, they'll just be automatically cleaned up on exit. Note that if a user is prompted for *other* downloads, they may still be taken to the chrome://downloads page and see the malicious download while they're there.
Added a new test in chrome/browser/lifetime/browser_close_manager_browsertest.cc.

BUG=306230, 305387
TEST=
1. Download a malicious file (e.g., http://download.safebrowsingtest.com/download/test)
2. Quit the browser
3. The browser should close without a warning
4. Download a dangerous file like a SWF
5. Quit the browser
6. The browser should pop up a dialog asking you whether you should save the file

R=asanka@chromium.org, benjhayden@chromium.org, joi@chromium.org, pkasting@chromium.org, thakis@chromium.org

Review URL: https://codereview.chromium.org/26938003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229784 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229784
Added the Prerender.ModPagespeedHeader.* histograms to histograms.xml. 
Extended the data extraction to also handle X-Page-Speed headers.
Supercedes 20832003 and 26279004.
[Adds a scoped_refptr to the unit test to stop a memory leak; actual functionality is unchanged]

BUG=267695

Review URL: https://codereview.chromium.org/26706002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@229785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 229785