Rolling libwebp broke our iOS builds. Silence warnings instead.

gyp/libwebp.gyp

Index: gyp/libwebp.gyp
diff --git a/gyp/libwebp.gyp b/gyp/libwebp.gyp
index 64ad971798b538e83c4b37be2fa1e660de90697b..f5fda6b65db5f97208e6843709a974e71e613ef7 100644
--- a/gyp/libwebp.gyp
+++ b/gyp/libwebp.gyp
@@ -36,6 +36,7 @@
             '../third_party/externals/libwebp/src/dec/webp.c',
           ],
           'cflags': [ '-w' ],
+          'xcode_settings': { 'WARNING_CFLAGS': [ '-w' ] },
         },
         {
           'target_name': 'libwebp_demux',
@@ -47,6 +48,7 @@
             '../third_party/externals/libwebp/src/demux/demux.c',
           ],
           'cflags': [ '-w' ],
+          'xcode_settings': { 'WARNING_CFLAGS': [ '-w' ] },
         },
         {
           'target_name': 'libwebp_dsp',
@@ -66,6 +68,7 @@
             '../third_party/externals/libwebp/src/dsp/yuv.c',
           ],
           'cflags': [ '-w' ],
+          'xcode_settings': { 'WARNING_CFLAGS': [ '-w' ] },
           'conditions': [
             ['skia_os == "android"', {
               'dependencies' : [
@@ -123,6 +126,7 @@
             '../third_party/externals/libwebp/src/enc/webpenc.c',
           ],
           'cflags': [ '-w' ],
+          'xcode_settings': { 'WARNING_CFLAGS': [ '-w' ] },
         },
         {
           'target_name': 'libwebp_utils',
@@ -144,6 +148,7 @@
             '../third_party/externals/libwebp/src/utils/utils.c',
           ],
           'cflags': [ '-w' ],
+          'xcode_settings': { 'WARNING_CFLAGS': [ '-w' ] },
         },
         {
           'target_name': 'libwebp',