[net] Disable fuzzer support targets unless use_fuzzing_engine is true.

Some buildbots are executing `ninja all` and fuzz targets should not
be attempted to be built there unless certain GN flags are enabled.

Bug: 999096
Change-Id: I11addbe7ab00a5ef8c57559bd84a1ec4b287f212
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1775756
Commit-Queue: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Matt Mueller <mattm@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691764}

net/BUILD.gn

@@ -6190,7 +6190,9 @@ if (!is_ios) {
 
 # This has a global (InitGlobals) that must always be linked in, so
 # must be a source set instead of a static library.
-if (!is_ios) {
+if (use_fuzzing_engine) {
+  # fuzzer_test targets are no-op when |use_fuzzing_engine| is false. Fuzzer
+  # support targets should be disabled too.
   source_set("net_fuzzer_test_support") {
     testonly = true
 

net/dns/BUILD.gn

@@ -453,17 +453,21 @@ source_set("test_support") {
   ]
 }
 
-source_set("fuzzer_test_support") {
-  testonly = true
-  sources = [
-    "fuzzed_host_resolver_util.cc",
-    "fuzzed_host_resolver_util.h",
-  ]
-  deps = [
-    "//base",
-    "//base/test:test_support",
-    "//net",
-  ]
+if (use_fuzzing_engine) {
+  # fuzzer_test targets are no-op when |use_fuzzing_engine| is false. Fuzzer
+  # support targets should be disabled too.
+  source_set("fuzzer_test_support") {
+    testonly = true
+    sources = [
+      "fuzzed_host_resolver_util.cc",
+      "fuzzed_host_resolver_util.h",
+    ]
+    deps = [
+      "//base",
+      "//base/test:test_support",
+      "//net",
+    ]
+  }
 }
 
 fuzzer_test("net_dns_hosts_parse_fuzzer") {

testing/libfuzzer/fuzzer_test.gni

@@ -34,7 +34,7 @@ import("//testing/test.gni")
 # config (.options file) file would be generated or modified in root output
 # dir (next to test).
 template("fuzzer_test") {
-  if (!disable_libfuzzer && (use_fuzzing_engine || is_linux)) {
+  if (!disable_libfuzzer && use_fuzzing_engine) {
     assert(defined(invoker.sources), "Need sources in $target_name.")
 
     test_deps = [ "//testing/libfuzzer:libfuzzer_main" ]

third_party/crashpad/README.chromium

@@ -36,3 +36,4 @@ $ git am --3way --message-id -p4 /tmp/patchdir
 
 Local Modifications:
  - codereview.settings has been excluded.
+ - elf_image_reader_fuzzer is enabled only when use_fuzzing_engine is true.

third_party/crashpad/crashpad/snapshot/BUILD.gn

@@ -236,7 +236,10 @@ static_library("snapshot") {
   }
 }
 
-if (crashpad_is_linux) {
+if (crashpad_is_linux && use_fuzzing_engine) {
+  # Since crashpad uses its own crashpad_fuzzer_test GN template to wrap
+  # Chromium's fuzzer_test template, it's better to rely on |use_fuzzing_engine|
+  # and disable fuzz targets as they will not build successfully without it.
   crashpad_fuzzer_test("elf_image_reader_fuzzer") {
     sources = [
       "elf/elf_image_reader_fuzzer.cc",