Add support to base::Timer for custom task runners.

This patch adds a public method
SetTaskRunner(scoped_refptr<SingleThreadTaskRunner>) to base::Timer
which changes the task runner used by the timer
(ThreadTaskRunnerHandler::Get() by default).

This change will enable scheduling timers on the queues of the
Blink scheduler (see https://codereview.chromium.org/637303003/).
Most importantly, we plan to apply this to the shared timer in
blink::Platform.

BUG=

Review URL: https://codereview.chromium.org/637983003

Cr-Commit-Position: refs/heads/master@{#302947}
diff --git a/build/get_landmines.py b/build/get_landmines.py
index 5740cb8..1eec5eec 100755
--- a/build/get_landmines.py
+++ b/build/get_landmines.py
@@ -60,6 +60,7 @@
   if platform() == 'android':
     print 'Delete stale generated .java files yet again. crbug.com/349592'
     print 'Clobber to delete incompatible object binary format with NDK r10c'
+  print 'Clobber to fix missing NaCl gyp dependencies (crbug.com/427427).'
 
 
 def main():