Build crash_service_win64 with clang-cl when is_clang=true

Otherwise this gn assertion fails:
  ERROR at //build/config/sanitizers/sanitizers.gni:85:1: Assertion failed.
  assert(!using_sanitizer || is_clang,
  ^-----
  Sanitizers (is_*san) require setting is_clang = true in 'gn args'

R=dpranke@chromium.org,scottmg@chromium.org
BUG=598761

Review URL: https://codereview.chromium.org/1838953003

Cr-Commit-Position: refs/heads/master@{#383808}

chrome/tools/crash_service/BUILD.gn

@@ -24,7 +24,13 @@ executable("crash_service") {
 if (target_cpu == "x86" && current_cpu == "x86") {
   # Cross-compile a 64-bit version when compiling a 64-bit target.
   copy("crash_service_win64") {
-    crash_service_64_target = ":crash_service(//build/toolchain/win:x64)"
+    if (is_clang) {
+      maybe_clang = "clang_"
+    } else {
+      maybe_clang = ""
+    }
+    crash_service_64_target =
+        ":crash_service(//build/toolchain/win:${maybe_clang}x64)"
 
     sources = [
       # It would be nice if get_target_outputs would work on the executable,