[LPM][libFuzzer][Windows]Enable building libprotobuf-mutator on Windows

Bug: 884560
Change-Id: I7e6ac6869b2e2e068214cc5944d73a22549e5a70
Reviewed-on: https://chromium-review.googlesource.com/1247189Reviewed-by: Max Moroz <mmoroz@chromium.org>
Commit-Queue: Jonathan Metzman <metzman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594710}

third_party/libprotobuf-mutator/BUILD.gn

@@ -10,9 +10,6 @@ config("include_config") {
 source_set("libprotobuf-mutator") {
   testonly = true
 
-  # Dont allow building on windows to avoid tryjob failure. This is OK since
-  # libFuzzer doesn't build on windows anyway.
-  if (!is_win) {
   configs += [ ":include_config" ]
 
   # Remove *San and coverage for a performance boost.
@@ -40,7 +37,6 @@ source_set("libprotobuf-mutator") {
   if (use_afl) {
     all_dependent_configs = [ "//testing/libfuzzer:no_clusterfuzz" ]
   }
-  }
 }
 
 # This protoc plugin, like the compiler, should only be built for the host