Flip enable_skia_wuffs_gif gn arg default to true

This is attempt chromium#2. Attempt chromium#1 (crrev.com/c/1898151) was rolled back
(crrev.com/c/1944286) due to performance and RAM usage regressions
(crbug.com/1023129, crbug.com/1023191).

During that time, there has been some optimization work done in the
upstream Skia project, especially around RAM usage, so we are trying
again.

Still, as crrev.com/c/1898151's commit message says, this change isn't
about performance per se, it is about tracking upstream Skia.
Nonetheless, here are some before-vs-after total times for Chromium's
"image_decode_bench -i 10" program for x86_64, on a selection of GIF
images that noel@chromium.org collects:

old_time  new_time  ratio file_name  file_size
0.000139  0.000325  0.43  bar.gif          265
0.000217  0.000315  0.69  radient.gif     1034
0.001144  0.001045  1.09  mondrian.gif    3792
0.002364  0.001987  1.19  rain.gif       10272
0.008795  0.005614  1.57  flag.gif       41738
0.020884  0.013044  1.60  warp.gif      106420
0.377886  0.286732  1.32  hex.gif      1072114

A ratio higher is better: more than 1.00 means that the new GIF decoder
is faster than the old one.

Bug: 1014044
Change-Id: Iba304db1ceea0661e0ca57308c2a68b2c48297a8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1958292
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Nigel Tao <nigeltao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724020}

skia/BUILD.gn

@@ -27,7 +27,11 @@ skia_support_pdf = !is_ios && enable_basic_printing
 skia_support_skottie = true
 
 declare_args() {
-  enable_skia_wuffs_gif = false
+  # If, in the future, this configuration option is deleted entirely, we can
+  # also probably delete any mention of the "libgifcodec" third party library
+  # from the top level DEPS and the third_party/.gitignore files.
+  enable_skia_wuffs_gif = true
+
   skia_whitelist_serialized_typefaces = false
 }