Add main world CSP bypass for isolated world iframe resource loads.

This adds support for bypass of the main world's CSP when an isolated world adds
an iframe to the page. This matches the behavior for images, scripts, styles,
etc. It is a slightly complicated case as when the resource actually loads, it
is no longer executing from the isolated world context since it is happening as
a task (even though it was created from an isolated world context).

BUG=386679
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/350653003

git-svn-id: svn://svn.chromium.org/blink/trunk@176898 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove #if ENABLE(OILPAN) from StyleEngine::m_documentStyleSheetCollection

This CL makes StyleEngine::m_documentStyleSheetCollection an OwnPtr in non-oilpan builds,
and fixes a FIXME in StyleEngine.h.

BUG=340522

Review URL: https://codereview.chromium.org/356503004

git-svn-id: svn://svn.chromium.org/blink/trunk@176899 bbb929c8-8fbe-4397-9dbb-9b2b20218538