Disallow multiple selection in desktop media picker.

Previously the picker was allowing multiple source selection. Now
DesktopMediaPickerController sets allowsMultipleSelection to NO in
IKImageBrowserView to disable this feature.

BUG=466997

Review URL: https://codereview.chromium.org/1128353005

Cr-Commit-Position: refs/heads/master@{#330453}

chrome/browser/ui/cocoa/media_picker/desktop_media_picker_controller.mm

@@ -127,6 +127,7 @@ const int kExcessButtonPadding = 6;
   [sourceBrowser_ setDataSource:self];
   [sourceBrowser_ setCellsStyleMask:cellStyle];
   [sourceBrowser_ setCellSize:NSMakeSize(kThumbnailWidth, kThumbnailHeight)];
+  [sourceBrowser_ setAllowsMultipleSelection:NO];
 
   // Create a scroll view to host the image browser.
   NSRect imageBrowserScrollFrame = NSMakeRect(