Relax HTMLBodyElement's scrollTop/Left property set and get operations.
After bug 303131 (r159035) and bug 157855 (r158864) are fixed, Blink started to
match other Web engines with regards to documentElement/body::scrollTop/Left
behaviors, according to the DocType mode. It turns out that these changes broken
some Web content relying on Blink quirks behavior.
That said, this CL takes a more conservative approach, where it was chosen to relax
the HTMLBodyElement bits of the CLs above, restoring their original behavior, but adding
an UseCount/DeprecationWarnings instead.
We are still more compliant with other Web engine with regards to documentElement's
scrollTop/Left operations, though.