Deprecate FileError in FileAPI

Apps should expect DOMError instead.

This change breaks existing apps, so I kept FileError in this CL
but made it inherit from DOMError and marked '.code' attribute
deprecated.

BUG=276861
TEST=fast/files/*,fast/filesystem/*

Review URL: https://chromiumcodereview.appspot.com/22831019

git-svn-id: svn://svn.chromium.org/blink/trunk@156562 bbb929c8-8fbe-4397-9dbb-9b2b20218538