Fix missing headers in libwebp

.gn

 # This file is used by the GN meta build system to find the root of the source
 # tree and to set startup options. For documentation on the values set in this
 # file, run "gn help dotfile" at the command line.
 
 import("//build/dotfile_settings.gni")
 
 # The location of the build configuration file.
 buildconfig = "//build/config/BUILDCONFIG.gn"
 
 # The secondary source root is a parallel directory tree where
@@ skipping 126 matching lines @@
   "//services/*",
   "//skia/*",
   "//sql/*",
   "//storage/*",
   "//testing/*",
 
   #"//third_party/*",  # May not ever want this.
   "//third_party/hunspell/*",
   "//third_party/libaddressinput/*",
   "//third_party/libphonenumber/*",
+  "//third_party/libwebp/*",
   "//third_party/WebKit/Source/*",
   "//tools/*",
 
   #"//ui/*",  # Work left on Chromeos w/ use_ozone. Some parts of UI that work:
   "//ui/accessibility/*",
   "//ui/android/*",
   "//ui/app_list/*",
   "//ui/arc/*",
   "//ui/aura/*",
   "//ui/aura_extra/*",
@@ skipping 117 matching lines @@
       "//third_party/google_input_tools/inputview.gni",
 
       # CLD2 should be removed soon, delete this when we do.
       "//third_party/cld_2/BUILD.gn",
       "//tools/grit/grit_rule.gni",
 
       # Not gypi-to-gn.
       "//google_apis/BUILD.gn",
       "//printing/BUILD.gn",
     ]