fix nacl build using ninja

gyp/common_conditions.gypi

 # conditions used in both common.gypi and skia.gyp in chromium
 #
 {
   'defines': [
     'SK_ALLOW_STATIC_GLOBAL_INITIALIZERS=<(skia_static_initializers)',
     'SK_SUPPORT_GPU=<(skia_gpu)',
     'SK_SUPPORT_OPENCL=<(skia_opencl)',
     'SK_FORCE_DISTANCEFIELD_FONTS=<(skia_force_distancefield_fonts)',
   ],
   'conditions' : [
@@ skipping 316 matching lines @@
             'defines': [
               'SKIA_DLL',
               'SKIA_IMPLEMENTATION=1',
             ],
           }],
           [ 'skia_os == "nacl"', {
             'defines': [
               'SK_BUILD_FOR_NACL',
             ],
             'variables': {
-              'nacl_sdk_root': '<!(["echo", "${NACL_SDK_ROOT}"])',
+              'nacl_sdk_root': '<!(echo ${NACL_SDK_ROOT})',
             },
             'link_settings': {
               'libraries': [
                 '-lppapi',
                 '-lppapi_cpp',
                 '-lnosys',
                 '-pthread',
               ],
               'ldflags': [
                 '-L<(nacl_sdk_root)/lib/newlib_x86_<(skia_arch_width)/Release',
@@ skipping 247 matching lines @@
       'defines': [
         'SK_USE_POSIX_THREADS',
       ],
     }],
   ], # end 'conditions'
   # The Xcode SYMROOT must be at the root. See build/common.gypi in chromium for more details
   'xcode_settings': {
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }