CompositingRequirementsUpdater should be ignorant of m_needsToRecomputeCompositingRequirements

The CompositingRequirementsUpdater is supposed to clean out the
m_needsToRecomputeCompositingRequirements dirtiness. The way the code is
written, it looks as if CompositingRequirementsUpdater can generate
m_needsToRecomputeCompositingRequirements dirtiness.

We have this code because
CompositingReasonFinder::requiresCompositingForPositionFixed can dirty
m_needsToRecomputeCompositingRequirements if layout information isn't yet
available. However, that can never happen when called from
CompositingRequirementsUpdater because CompositingRequirementsUpdater is called
after all the layout state is cleaned.

After this CL, we now crash if CompositingRequirementsUpdater tries to generate
dirtiness in m_needsToRecomputeCompositingRequirements.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/269023007

git-svn-id: svn://svn.chromium.org/blink/trunk@173355 bbb929c8-8fbe-4397-9dbb-9b2b20218538