Roll libwebp to v0.4.2 (latest stable) to fix annoying build warning.

gyp/libwebp.gyp

Index: gyp/libwebp.gyp
diff --git a/gyp/libwebp.gyp b/gyp/libwebp.gyp
index 64ad971798b538e83c4b37be2fa1e660de90697b..762c3b7b1e1461a72ecb1667141b999a776fa705 100644
--- a/gyp/libwebp.gyp
+++ b/gyp/libwebp.gyp
@@ -28,7 +28,6 @@
             '../third_party/externals/libwebp/src/dec/frame.c',
             '../third_party/externals/libwebp/src/dec/idec.c',
             '../third_party/externals/libwebp/src/dec/io.c',
-            '../third_party/externals/libwebp/src/dec/layer.c',
             '../third_party/externals/libwebp/src/dec/quant.c',
             '../third_party/externals/libwebp/src/dec/tree.c',
             '../third_party/externals/libwebp/src/dec/vp8.c',
@@ -55,15 +54,20 @@
               '../third_party/externals/libwebp',
           ],
           'sources': [
+            '../third_party/externals/libwebp/src/dsp/alpha_processing.c',
+            '../third_party/externals/libwebp/src/dsp/alpha_processing_sse2.c',
             '../third_party/externals/libwebp/src/dsp/cpu.c',
             '../third_party/externals/libwebp/src/dsp/dec.c',
+            '../third_party/externals/libwebp/src/dsp/dec_clip_tables.c',
             '../third_party/externals/libwebp/src/dsp/dec_sse2.c',
             '../third_party/externals/libwebp/src/dsp/enc.c',
             '../third_party/externals/libwebp/src/dsp/enc_sse2.c',
             '../third_party/externals/libwebp/src/dsp/lossless.c',
+            '../third_party/externals/libwebp/src/dsp/lossless_sse2.c',
             '../third_party/externals/libwebp/src/dsp/upsampling.c',
             '../third_party/externals/libwebp/src/dsp/upsampling_sse2.c',
             '../third_party/externals/libwebp/src/dsp/yuv.c',
+            '../third_party/externals/libwebp/src/dsp/yuv_sse2.c',
           ],
           'cflags': [ '-w' ],
           'conditions': [
@@ -85,6 +89,7 @@
               'sources': [
                 '../third_party/externals/libwebp/src/dsp/dec_neon.c',
                 '../third_party/externals/libwebp/src/dsp/enc_neon.c',
+                '../third_party/externals/libwebp/src/dsp/lossless_neon.c',
                 '../third_party/externals/libwebp/src/dsp/upsampling_neon.c',
               ],
               # behavior similar dsp_neon.c.neon in an Android.mk
@@ -113,8 +118,11 @@
             '../third_party/externals/libwebp/src/enc/frame.c',
             '../third_party/externals/libwebp/src/enc/histogram.c',
             '../third_party/externals/libwebp/src/enc/iterator.c',
-            '../third_party/externals/libwebp/src/enc/layer.c',
             '../third_party/externals/libwebp/src/enc/picture.c',
+            '../third_party/externals/libwebp/src/enc/picture_csp.c',
+            '../third_party/externals/libwebp/src/enc/picture_psnr.c',
+            '../third_party/externals/libwebp/src/enc/picture_rescale.c',
+            '../third_party/externals/libwebp/src/enc/picture_tools.c',
             '../third_party/externals/libwebp/src/enc/quant.c',
             '../third_party/externals/libwebp/src/enc/syntax.c',
             '../third_party/externals/libwebp/src/enc/token.c',
@@ -139,6 +147,7 @@
             '../third_party/externals/libwebp/src/utils/huffman_encode.c',
             '../third_party/externals/libwebp/src/utils/quant_levels.c',
             '../third_party/externals/libwebp/src/utils/quant_levels_dec.c',
+            '../third_party/externals/libwebp/src/utils/random.c',
             '../third_party/externals/libwebp/src/utils/rescaler.c',
             '../third_party/externals/libwebp/src/utils/thread.c',
             '../third_party/externals/libwebp/src/utils/utils.c',