Code implements UI for downloading and burning Chrome OS images on SSD card...

Code implements UI for downloading and burning Chrome OS images on SSD card and USB key. Actual burning is not included in the change list.

BUG=chromium-os:5346

TEST=type in  chrome://imagebuner in browser. UI lists all media that image can be burnt to. After selecting burning target by clicking an image right to the target name, image download should start, and download progress should be displayed. After download ends alert should pop up asking user to confirm that he wants to burn image to selected device. Clicking both ok or cancel shouldn't do anything, since actual burning isn't stil included in CL.
Image should be downloaded to chrome_image folder in users Downloads directory. 
this folder is deleted during shutdown.
This is only visible in ChromeOS...

Review URL: http://codereview.chromium.org/2808100

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@57085 0039d316-1c4b-4281-b951-d872f2087c98