libwebp,BUILD: add WEBP_REDUCE_SIZE

removes rescaler, auto-filter (-af) support and makes WebPPictureCopy,
WebPPictureIsView, WebPPictureView, WebPPictureCrop, and
WebPPictureRescale noops.

Bug: 789665
Change-Id: I222feb7dce9b41093acebf1d3381dcedf9922e7d
Reviewed-on: https://chromium-review.googlesource.com/797771Reviewed-by: Urvang Joshi <urvang@chromium.org>
Commit-Queue: James Zern <jzern@google.com>
Cr-Commit-Position: refs/heads/master@{#521944}

third_party/libwebp/BUILD.gn

@@ -15,6 +15,7 @@ config("libwebp_config_internal") {
   defines = [
     "WEBP_NEAR_LOSSLESS=0",
     "WEBP_DISABLE_STATS",
+    "WEBP_REDUCE_SIZE",
   ]
   if (!is_ios) {
     defines += [ "WEBP_REDUCE_CSP" ]