Move testRunner.startSpeechInput/setValueForUser to internals

Now TestRunner is being moved from CppBoundClass to gin::Wrappable,
where it is not appropriate to use WebElements directly. This CL moves
some functions which treats WebElement* from testRunner to internals.
(To be exact, this CL only copies some functions, but I'll remove those ones
in testRunner later.)

BUG=331301

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=168495

Review URL: https://codereview.chromium.org/176953009

git-svn-id: svn://svn.chromium.org/blink/trunk@168588 bbb929c8-8fbe-4397-9dbb-9b2b20218538