[Files.app] Fetch and use CWS OAuth token instead of Drive token in Suggest app dialog.

We need to pass the CWS OAuth token to the CWS widget in Suggest app dialog, instead of Drive OAuth token. This patch adds the method to fetch the CWS token and replace the drive token with it in Suggest app dialog.

BUG=none
TEST=manual

Review URL: https://chromiumcodereview.appspot.com/23549016

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@222004 0039d316-1c4b-4281-b951-d872f2087c98

• chrome/browser/chromeos/extensions/file_manager/private_api_misc.cc[diff]
• chrome/browser/chromeos/extensions/file_manager/private_api_misc.h[diff]
• chrome/browser/extensions/extension_function_histogram_value.h[diff]
• chrome/browser/resources/file_manager/js/suggest_apps_dialog.js[diff]
• chrome/common/extensions/api/file_browser_private.json[diff]