Implement main blink target in GN.

Source/platform/heap/BUILD.gn

 # Copyright 2014 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.
 
+visibility = "//third_party/WebKit/Source/*"
+
 source_set("heap") {
   sources = [
     "AddressSanitizer.h",
     "Handle.h",
     "Heap.cpp",
     "Heap.h",
     "ThreadState.cpp",
     "ThreadState.h",
     "Visitor.cpp",
     "Visitor.h",
   ]
   if (cpu_arch == "x86" || cpu_arch == "x64") {
     sources += [ "asm/SaveRegisters_x86.asm" ]
   } else if (cpu_arch == "arm") {
     sources += [ "asm/SaveRegisters_arm.S" ]
   } else if (cpu_arch == "arm64") {
     sources += [ "asm/SaveRegisters_arm64.S" ]
   } else if (cpu_arch == "mipsel") {
     sources += [ "asm/SaveRegisters_mips.S" ]
   }
 
   configs += [ "//third_party/WebKit/Source:config" ]
 
   deps = [
     "//third_party/icu",
   ]
 }