Give signin team ownership of chrome.identity extension API impl

Change-Id: I2e894379abc801ef364a9228b89e3cc23eaf408c
Reviewed-on: https://chromium-review.googlesource.com/968247
Commit-Queue: Colin Blundell <blundell@chromium.org>
Reviewed-by: Mihai Sardarescu <msarda@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544009}

chrome/browser/extensions/api/identity/OWNERS

+droger@chromium.org
 msarda@chromium.org
 
 # Original author, very slow on reviews as no longer working on Chromium.
 courage@chromium.org
+
+# TEAM: chrome-signin@chromium.org
+# COMPONENT: Services>SignIn