GCC: do not use blink heap fuzzed data provider if build is not using clang

GCC build of blink_heap_unittests is broken as it requires the fuzzed data
provider. But libfuzzer is not available without clang.

Bug: 819294
Change-Id: I377ed09d03fd8bdc2561d766a1964d4ab61943dc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1901038Reviewed-by: Martin Barbella <mbarbella@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Commit-Queue: José Dapena Paz <jose.dapena@lge.com>
Cr-Commit-Position: refs/heads/master@{#713190}

third_party/blink/renderer/platform/BUILD.gn

@@ -1662,7 +1662,7 @@ jumbo_static_library("test_support") {
   ]
 
   # fuzzed_data_provider may not work with a custom toolchain.
-  if (custom_toolchain == "") {
+  if (custom_toolchain == "" && is_clang) {
     sources += [
       "testing/fuzzed_data_provider.cc",
       "testing/fuzzed_data_provider.h",