Change Wallpaper Private API to be able to search on MyFiles

MyFiles is being changed to allow users to create files/folders, this CL
changes Wallpaper to be able to scan images in MyFiles instead of
Downloads.
The function |MyFilesFolderForProfile| takes care of returning Downloads
path if the flag MyFilesVolume is disabled.

Tested manually by loading in a device with MyFilesVolume flag enabled.

Bug: 873539
Change-Id: If0e40cd1d2961ec5ca380a799c5a272823c0cede
Reviewed-on: https://chromium-review.googlesource.com/c/1353031Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Commit-Queue: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#612426}

chrome/browser/chromeos/extensions/wallpaper_private_api.cc

@@ -723,7 +723,7 @@ WallpaperPrivateGetLocalImagePathsFunction::
 
 ExtensionFunction::ResponseAction
 WallpaperPrivateGetLocalImagePathsFunction::Run() {
-  base::FilePath path = file_manager::util::GetDownloadsFolderForProfile(
+  base::FilePath path = file_manager::util::GetMyFilesFolderForProfile(
       Profile::FromBrowserContext(browser_context()));
   base::PostTaskAndReplyWithResult(
       WallpaperFunctionBase::GetNonBlockingTaskRunner(), FROM_HERE,