Initialize ClipRectsCache::m_clipRectsRoot

In the previous CL, I moved ClipRectsCache::m_clipRectsRoot from being a
debug-only member to being present in release builds. However, I forgot to move
the code that initializes the member into the release build.

BUG=369141

Review URL: https://codereview.chromium.org/261043002

git-svn-id: svn://svn.chromium.org/blink/trunk@173106 bbb929c8-8fbe-4397-9dbb-9b2b20218538