Set gtest_enable_absl_printers to true.

Starting from [1], gtest can pretty print absl types. In order to
enable the feature WebRTC has to set gtest_enable_absl_printers to true
in the .gn file.

[1] - https://chromium-review.googlesource.com/c/chromium/src/+/1027711

Bug: None
Change-Id: I74eb9a48c361f1523dd8d45510297e101a4d14cd
Reviewed-on: https://webrtc-review.googlesource.com/85345
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#23781}

.gn

@@ -83,4 +83,6 @@ default_args = {
   # WebRTC does not provide the gflags dependency. Because libyuv uses it only
   # for unittests, it can be disabled (see third_party/libyuv/BUILD.gn)
   libyuv_use_gflags = false
+
+  gtest_enable_absl_printers = true
 }