Move bit_cast from base/macros.h to its own header

base/BUILD.gn

 # Copyright (c) 2013 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.
 
 # HOW TO WRITE CONDITIONALS IN THIS FILE
 # ======================================
 #
 # In many other places, one would write a conditional that expresses all the
 # cases when a source file is used or unused, and then either add or subtract
 # it from the sources list in that case
@@ skipping 200 matching lines @@
     "base64url.h",
     "base_export.h",
     "base_switches.h",
     "big_endian.cc",
     "big_endian.h",
     "bind.h",
     "bind_helpers.cc",
     "bind_helpers.h",
     "bind_internal.h",
     "bind_internal_win.h",
+    "bit_cast.h",
     "bits.h",
     "build_time.cc",
     "build_time.h",
     "callback.h",
     "callback_helpers.cc",
     "callback_helpers.h",
     "callback_internal.cc",
     "callback_internal.h",
     "cancelable_callback.h",
     "command_line.cc",
@@ skipping 1915 matching lines @@
 
   # GYP: //base.gyp:base_java_unittest_support
   android_library("base_java_unittest_support") {
     deps = [
       ":base_java",
     ]
     java_files =
         [ "test/android/java/src/org/chromium/base/ContentUriTestUtils.java" ]
   }
 }