Expose FrameLoader::m_currentItem, use it in core/

Also, move call to HistoryController::setDefersLoading from FrameLoader to
Page, since HistoryController is per-Page rather than per-Frame now.

BUG=321280

Review URL: https://codereview.chromium.org/76783002

git-svn-id: svn://svn.chromium.org/blink/trunk@162428 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Modify some ASSERTs for bad casts so that ASSERT_WITH_SECURITY_IMPLICATION is triggered instead.

BUG=286464

Review URL: https://codereview.chromium.org/77733008

git-svn-id: svn://svn.chromium.org/blink/trunk@162429 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Move ContextMenu* files from core/ to platform/

BUG=297477
TEST=None, no functional changes.
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/78513004

git-svn-id: svn://svn.chromium.org/blink/trunk@162430 bbb929c8-8fbe-4397-9dbb-9b2b20218538