Move CSSRuleCollection class to its own file

BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build_overrides/v8.gni")
 import("//testing/test.gni")
 import("pdfium.gni")
 
 config("pdfium_common_config") {
   cflags = []
@@ skipping 923 matching lines @@
       "xfa/fde/cfde_txtedtpage.cpp",
       "xfa/fde/cfde_txtedtpage.h",
       "xfa/fde/cfde_txtedtparag.cpp",
       "xfa/fde/cfde_txtedtparag.h",
       "xfa/fde/cfde_txtedttextset.cpp",
       "xfa/fde/cfde_txtedttextset.h",
       "xfa/fde/cfx_chariter.cpp",
       "xfa/fde/cfx_chariter.h",
       "xfa/fde/cfx_wordbreak.cpp",
       "xfa/fde/cfx_wordbreak.h",
+      "xfa/fde/css/cfde_cssrulecollection.cpp",
       "xfa/fde/css/fde_css.cpp",
       "xfa/fde/css/fde_css.h",
       "xfa/fde/css/fde_csscache.cpp",
       "xfa/fde/css/fde_csscache.h",
       "xfa/fde/css/fde_cssdatatable.cpp",
       "xfa/fde/css/fde_cssdatatable.h",
       "xfa/fde/css/fde_cssdeclaration.cpp",
       "xfa/fde/css/fde_cssdeclaration.h",
       "xfa/fde/css/fde_cssstyleselector.cpp",
       "xfa/fde/css/fde_cssstyleselector.h",
@@ skipping 630 matching lines @@
 }
 
 if (pdf_is_standalone) {
   source_set("samples") {
     testonly = true
     deps = [
       "//samples",
     ]
   }
 }