Fix print preview dialog for <webview>.

Fix print preview dialog for <webview>.

Make constrained window use embedder's native window
(through its WebContents) instead of using webview's.

After print preview got enabled by default, the code
trying to bring up print preview dialog (a constrained
window) started crashing the browser.

BUG=286487
Test=In chromeos device/LinuxAura/Mac: load the sample
browser app
https://github.com/GoogleChrome/chrome-app-samples/tree/master/webview-samples/browser
Navigate to a page that calls window.print()
http://www.w3schools.com/jsref/tryit.asp?filename=tryjsref_print
Click "Print this page" to trigger window.print() inside
webview.
It should
1) not crash
2) correctly show the preview and print the contents of
the webview, not the entire app. This
means it shouldn't print the chrome (address bar/buttons etc.)
of the browser sample.

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=276372

[lazyboy, 2014-06-10 20:22:14]

Fady, can you do an initial review?
Specially for the code pattern:
WebViewGuest::FromWebContents(wc)->embedder_web_contents();

I'll ask sky@ for review once you've taken a look.

[Fady Samuel, 2014-06-10 21:16:39]

lgtm from a webview perspective.

[lazyboy, 2014-06-10 21:21:22]

+sky for review.

[sky, 2014-06-10 22:19:56]

LGTM

[lazyboy, 2014-06-11 05:09:16]

The CQ bit was checked by lazyboy@chromium.org

[commit-bot: I haz the power, 2014-06-11 05:10:31]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/lazyboy@chromium.org/321193003/80001

[commit-bot: I haz the power, 2014-06-11 12:56:47]

Change committed as 276372