Remove broken code for yielding parser to layout

This code previously was attempting to yield to layout
before running scripts.  However now that layout and style
are no longer timer-based in Blink but rather driven by
the compositor's serviceAnimations callback, we no longer
can yield to them in this way.  Especially since the
compositor will only call us to layout just before
putting up a frame, which it will never do for a background
tab.  However a background tab can still continue to
run its parser, which with this code would constantly
attempt to defer itself by 0ms indefinitely.

The parser yielding logic really no longer makes
any sense at all, since it only ever yields to the
main thread by scheduling a 0ms timer which
will be run again immediately (before returning
to the chromium event loop) so long as we haven't
yet exceeded SharedTimer::maxDurationOfFiringTimers
which is currently set to 50ms.

Parser yielding, even when working is only managing to
return after 50ms while there is still work to do.

Unfortunately I know of no good way to test this
since we currently don't have any way to disable the
(fake) compositor callbacks in DumpRenderTree.

I have tested loading the hanging page:
http://clothing.shop.ebay.com/Womens-Shoes-/63889/i.html
in a hundred background tabs and am no longer able
to trigger the 100% cpu usage spin.

BUG=288265

Review URL: https://codereview.chromium.org/229443003

git-svn-id: svn://svn.chromium.org/blink/trunk@171097 bbb929c8-8fbe-4397-9dbb-9b2b20218538