Don't call body() so often in inheritHtmlAndBodyElementStyles()

body() contains a loop over the children of the document element, which
in the common case is the <head> and then the <body>, so this code was
executing a for loop to skip over the <head> and do a bodyTag check
repeatedly. Instead we can just keep the body on the stack.

I also removed a !frame() and !view() check which weren't needed. Those
things are already ASSERT'ed about by ::recalcStyle which calls this,
and also added an ASSERT for the documentElement() by moving the call
to inheritHtmlAndBodyElementStyles in Document::recalcStyle.

Review URL: https://codereview.chromium.org/64913007

git-svn-id: svn://svn.chromium.org/blink/trunk@161817 bbb929c8-8fbe-4397-9dbb-9b2b20218538