Add testRunner.insertStyleSheet for testing style sheet injection.

Add testRunner.insertStyleSheet for testing style sheet injection.

testRunner.injectStyleSheet uses a different code path from the only way
to inject stylesheets in Chrome (extensions) now that user stylesheets
are no longer supported. In addition, the Blink implementation is not
friendly to OOPI, since it expects all frames to be in one process.

Tying it to WebLocalFrame::frameForCurrentContext() is a bit awkward,
but all the TestRunner plumbing currently expects to work on a
per-WebView basis, and changing it is rather complicated. Once a
TestRunner can be associated with a WebFrame, it won't need to be tied
to the current context and the corresponding layout test can be cleaned
up.

BUG=

Review URL: https://codereview.chromium.org/1056913002

Cr-Commit-Position: refs/heads/master@{#323646}