Clear ScratchBuffer::m_lastLayerSize when clearing the scratch buffer.

Clear ScratchBuffer::m_lastLayerSize when clearing the scratch buffer.
https://bugs.webkit.org/show_bug.cgi?id=126150

Reviewed by Simon Fraser.

Since ScratchBuffer::clearScratchBuffer only clears m_lastRadius,
ShadowBlur doesn't draw shadow into the re-created scratch buffer if it
tries to draw shadow without blurRadius.

Clear m_lastLayerSize to empty is enought to ensure that there is no
drawn contents in the scratch buffer.

No new tests due to the flaky nature of reproducing the issue.

* platform/graphics/ShadowBlur.cpp:
(WebCore::ScratchBuffer::clearScratchBuffer):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161014 268f45cc-cd09-0410-ab3c-d52691b4dbfc