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=

Committed: https://crrev.com/9000dbd4ec56fc35123ae257c51981697a1c8e28
Cr-Commit-Position: refs/heads/master@{#323646}

[dcheng, 2015-04-02 15:55:10]

dcheng@chromium.org changed reviewers:
+ pfeldman@chromium.org

[dcheng, 2015-04-02 15:55:11]

Context: this is needed to unblock https://codereview.chromium.org/1058723002
from landing, which will unblock the removal of the old stylesheet injection
code.

[pfeldman, 2015-04-02 18:46:54]

On 2015/04/02 15:55:11, dcheng wrote:
> Context: this is needed to unblock https://codereview.chromium.org/1058723002
> from landing, which will unblock the removal of the old stylesheet injection
> code.

Oh, awesome, somehow I got confused and thought it would need to go through the
plumbing. lgtm.

[dcheng, 2015-04-02 19:26:02]

The CQ bit was checked by dcheng@chromium.org

[commit-bot: I haz the power, 2015-04-02 19:28:48]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1056913002/1

[commit-bot: I haz the power, 2015-04-03 05:39:46]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-04-03 20:33:19]

Patchset 1 (id:??) landed as
https://crrev.com/9000dbd4ec56fc35123ae257c51981697a1c8e28
Cr-Commit-Position: refs/heads/master@{#323646}