Clipboard Android - Store and Read Last Modified Time from Prefs

Tested by enabling clipboard provider and the clipboard interactively on a device.
Clipboard suggestions are persisted to and restored from prefs, which works
even if I kill Chrome.
I also added LOG(INFO) lines during this and saw the writes and reads happening
at the correct time and with the right values.

BUG=711574
[email protected], [email protected]

Review-Url: https://codereview.chromium.org/2832263002 .
Cr-Commit-Position: refs/heads/master@{#467796}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index 924a943d..c96f62da 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -2932,6 +2932,8 @@
       "android/preferences/autofill/autofill_profile_bridge.h",
       "android/preferences/browser_prefs_android.cc",
       "android/preferences/browser_prefs_android.h",
+      "android/preferences/clipboard_android.cc",
+      "android/preferences/clipboard_android.h",
       "android/preferences/pref_service_bridge.cc",
       "android/preferences/pref_service_bridge.h",
       "android/preferences/preferences_launcher.cc",