Merge: [fsp] Promote file system provider to stable.

This patch cleans up the IDL and promotes the API to stable.

TBR=fukino@chromium.org
TEST=browser_tests: *FileSystemProvider*
BUG=248427

Review URL: https://codereview.chromium.org/706153002

Cr-Commit-Position: refs/heads/master@{#303975}
(cherry picked from commit 4d752f9dc097c5f1f08e610aabbe2abf475d4072)

Review URL: https://codereview.chromium.org/810213002

Cr-Commit-Position: refs/branch-heads/2214@{#324}
Cr-Branched-From: 03655fd3f6d72165dc3c9bd2c89807305316fe6c-refs/heads/master@{#303346}
5 files changed