Fix missing negation on early exit condition when computing position constraints

In r159907 a small mistake landed where I missed a negation operator.
By fluke, the negation operator does not actually affect
fixed-position correctness: either the composited layer will be
implicitly fixed to another composited layer that is truly fixed (the
original way the code intended to work), or it will walk further up
the tree and be fixed with respect to the actual fixed-position
element. There may have been a minor performance hit, though,
with the mistake, because of additional position constraints in the
layer tree that need costly scroll compensation computations.

Pinch-zoom on the impl thread may behave differnetly, since a layer may
inherit awkward position constraints that should not have applied. But
there does not seem to be any reliable way to test that in a layout test
on the blink side.

BUG=317881

Review URL: https://codereview.chromium.org/70103002

git-svn-id: svn://svn.chromium.org/blink/trunk@161825 bbb929c8-8fbe-4397-9dbb-9b2b20218538