Remove implicit conversions from scoped_refptr to T* in cc/trees/

This patch was generated by running the rewrite_scoped_refptr clang tool
on a Linux build.

BUG=110610
R=danakj@chromium.org

Review URL: https://codereview.chromium.org/500123003

Cr-Commit-Position: refs/heads/master@{#291807}
12 files changed