Use a ResizeObserver to determine default font-size for text tracks

third_party/WebKit/Source/core/layout/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("//third_party/WebKit/Source/core/core.gni")
 
 blink_core_sources("layout") {
   split_count = 5
 
   sources = [
@@ skipping 166 matching lines @@
     "LayoutTextCombine.cpp",
     "LayoutTextCombine.h",
     "LayoutTextControl.cpp",
     "LayoutTextControl.h",
     "LayoutTextControlMultiLine.cpp",
     "LayoutTextControlMultiLine.h",
     "LayoutTextControlSingleLine.cpp",
     "LayoutTextControlSingleLine.h",
     "LayoutTextFragment.cpp",
     "LayoutTextFragment.h",
-    "LayoutTextTrackContainer.cpp",
-    "LayoutTextTrackContainer.h",
     "LayoutTheme.cpp",
     "LayoutTheme.h",
     "LayoutThemeDefault.cpp",
     "LayoutThemeDefault.h",
     "LayoutThemeFontProvider.cpp",
     "LayoutThemeFontProvider.h",
     "LayoutThemeMac.h",
     "LayoutThemeMac.mm",
     "LayoutThemeMobile.cpp",
     "LayoutThemeMobile.h",
@@ skipping 267 matching lines @@
       "LayoutThemeAndroid.cpp",
       "LayoutThemeAndroid.h",
     ]
   }
 
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",
   ]
 }