Remove overly aggressive assert.

These asserts were firing due to a number of Persistent<CSSValue>'s left at the time we tear down the main thread due to them being RefCountedGarbageCollected which creates a persistent.
I am removing the asserts since we cannot guarantee there are no RefPtr's to e.g. a CSSValue when tearing down the main thread at this point in time. Running the current version of webkit_unit_tests shows we are leaking quite a few objects:

<webkit_unit_tests output>

LEAK: 7 RenderObject
LEAK: 1 XMLHttpRequest
LEAK: 3 Page
LEAK: 2 Frame
LEAK: 96 Resource
LEAK: 1 Range
LEAK: 4210 WebCoreNode

</webkit_unit_tests output>

R=ager@chromium.org, erik.corry@gmail.com, haraken@chromium.org, vegorov@chromium.org, zerny@chromium.org
BUG=

Review URL: https://codereview.chromium.org/158893004

git-svn-id: svn://svn.chromium.org/blink/trunk@166906 bbb929c8-8fbe-4397-9dbb-9b2b20218538