Revert 226311 "ChildProcessSecurityPolicy: Rationalize PlatformF..."

> ChildProcessSecurityPolicy: Rationalize PlatformFileFlags used.
> 
> Eventually I want to move from PlatformFileFlags to a custom set of flags. Before that, here is an attempted rationalization and unification with the PlatformFileFlags defined in fileapi/file_permission_policy. This is so I have a firm basis to work off of.
> 
> Depends on:
> https://codereview.chromium.org/24406003/
> 
> This is part of Step 4 of this refactoring plan:
> https://docs.google.com/a/google.com/document/d/1QGkGWuwgSuaRqovz4wyb0upqPKDVsgYOFKt44E7gmOE/edit?usp=sharing
> 
> BUG=262142, 263150
> 
> Review URL: https://codereview.chromium.org/24631002

TBR=tommycli@chromium.org

Review URL: https://codereview.chromium.org/25616003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226316 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226316