libwebp: update to 0.4.2-rc2

third_party/libwebp/BUILD.gn

 # Copyright 2014 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.
 
 import("//build/config/arm.gni")
 
 config("libwebp_config") {
   include_dirs = [ "." ]
 }
 
@@ skipping 38 matching lines @@
     ":libwebp_config"
   ]
   configs -= [ "//build/config/compiler:chromium_code" ]
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }
 
 
 source_set("libwebp_dsp") {
   sources = [
     "dsp/alpha_processing.c",
+    "dsp/alpha_processing_sse2.c",
     "dsp/cpu.c",
     "dsp/dec.c",
     "dsp/dec_clip_tables.c",
     "dsp/dec_mips32.c",
     "dsp/dec_sse2.c",
     "dsp/enc.c",
     "dsp/enc_avx2.c",
     "dsp/enc_mips32.c",
     "dsp/enc_sse2.c",
     "dsp/lossless.c",
@@ skipping 124 matching lines @@
     ":libwebp_enc",
     ":libwebp_utils",
   ]
   public_configs = [
     ":libwebp_config"
   ]
   if (use_dsp_neon) {
     deps += [ ":libwebp_dsp_neon" ]
   }
 }