Remove usage of {,un}setPendingActivity from SpeechRecognition

{,un}setPendingActivity relies on RefCounted, which is going to be deprecated in oilpan. This CL removes usage of these methods in SpeechRecognition.cpp, and provide hasPendingActivity() implementation as replacement

BUG=354845
TEST=fast/speech

Review URL: https://codereview.chromium.org/208273002

git-svn-id: svn://svn.chromium.org/blink/trunk@169746 bbb929c8-8fbe-4397-9dbb-9b2b20218538