Add comment about why blink_tests is not in third_party/blink

Attemps to clarify the reason why blink_tests is not in third_party/blink

Change-Id: Id7938c8259a61fab5485f4c1f48a772abf59a62c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1874967Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710109}

BUILD.gn

@@ -937,7 +937,10 @@ group("gn_visibility") {
 
 if (!is_ios) {
   # This group includes all of the targets needed to build and test Blink,
-  # including running the layout tests (see below).
+  # including running web tests (see below). This target is defined here because
+  # previously //third_party/WebKit, now //third_party/blink, couldn't depend on
+  # //content/shell. This might not longer be true, see
+  # https://crbug.com/1018659.
   group("blink_tests") {
     testonly = true