Make ExecutionContext::setClient() protected

It's called only by its subclasses.

BUG=none
R=haraken

Review URL: https://codereview.chromium.org/324003003

git-svn-id: svn://svn.chromium.org/blink/trunk@175863 bbb929c8-8fbe-4397-9dbb-9b2b20218538