Add WebKitUnitTestSupport::readFromFile and deprecate all PlatformFileHandle related methods

- Add WebKitUnitTestSupport::readFromFile
- Remove WebFileUtilities::{openFile,closeFile,readFromFile}

BUG=267341

Review URL: https://chromiumcodereview.appspot.com/22645008

git-svn-id: svn://svn.chromium.org/blink/trunk@155941 bbb929c8-8fbe-4397-9dbb-9b2b20218538