[GN] Add support for disabling opts via SK_BUILD_NO_OPTS define.

gyp/common_conditions.gypi

 # Copyright 2015 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
@@ skipping 544 matching lines @@
 
             '-Wno-unused-parameter',
           ],
         },
       },
     ],
 
     [ 'skia_os == "ios"',
       {
         'defines': [
+          # When targetting iOS and using gyp to generate the build files, it is
+          # not possible to select files to build depending on the architecture
+          # (i.e. it is not possible to use hand optimized assembly version). In
+          # that configuration, disable all optimisation.
           'SK_BUILD_FOR_IOS',
+          'SK_BUILD_NO_OPTS',
         ],
         'conditions' : [
           [ 'skia_warnings_as_errors', {
             'xcode_settings': {
               'OTHER_CPLUSPLUSFLAGS': [
                 '-Werror',
               ],
             },
           }],
         ],
@@ skipping 113 matching lines @@
           'LD_RUNPATH_SEARCH_PATHS': ['@executable_path/.'],
       },
     }],
 
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }