Remove implicit conversions from scoped_refptr to T* in c/b/chromeos/file*

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

BUG=110610

Review URL: https://codereview.chromium.org/564503002

Cr-Commit-Position: refs/heads/master@{#294309}
7 files changed