Split nonstandard File Writer/File System API parts out of externs/browser/fileapi.js

fileapi.js contains externs for:
- [File API](https://github.com/w3c/FileAPI) - actively maintained specification, implemented in all browsers
- [File API: Writer](https://www.w3.org/TR/file-writer-api/) - working group note, only implemented in Chrome
- [File API: Directories and System](https://www.w3.org/TR/file-system-api/) - working group note, only implemented in Chrome
- [Quota Management API](https://www.w3.org/TR/quota-api/) - working group note, only implemented in Chrome

Definitions from the latter three should be split out as browser-specific. (Although portions may end up being de-facto standards, see http://wicg.github.io/entries-api/)

In addition:
- The [File API](https://github.com/w3c/FileAPI) externs reference the type `FileError` which is not in the specification and not implemented in browsers other than Chrome.
- The externs include types that have been removed by browsers, such as `BlobBuilder`

Looking at the top level function definitions, the split up should probably be:
- File API:
- Blob
- File
- FileReader
- createObjectURL
- revokeObjectURL
- Chrome-specific externs:
- DirectoryEntry
- DirectoryReader
- Entry
- FileEntry
- FileSystem
- FileWriter
- Metadata
- webkitRequestFileSystem
- StorageInfo
- StorageQuota
- Remove:
- BlobBuilder
- WebKitBlobBuilder
- FileError
- FileSaver
- LocalFileSystem
- requestFileSystem
- resolveLocalFileSystemURI
- webkitResolveLocalFileSystemURI
- webkitURL