Re-enable SitePerProcessBrowserTest.CrossSiteIframe

This CL fixes a few issues in the cross-process subframe navigation and re-enables the CrossSiteIframe test on most platforms. There are still some issues on ChromeOS and Android, which will be investigated and fixed in follow-up CL.

BUG=399775, 357747

Review URL: https://codereview.chromium.org/479403004

Cr-Commit-Position: refs/heads/master@{#291437}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291437 0039d316-1c4b-4281-b951-d872f2087c98
16 files changed