Make SpeechSynthesis gracefully handle ExecutionContext-less operation.

The SpeechSynthesis may lose its observed execution context and still
be accessed by the platform. Check if present before accessing the
context.

R=haraken@chromium.org
BUG=345061

Review URL: https://codereview.chromium.org/330603002

git-svn-id: svn://svn.chromium.org/blink/trunk@176018 bbb929c8-8fbe-4397-9dbb-9b2b20218538