[tint] Don't link 'benchmark_main' target

Tint provides its own main().

Also add the benchmark target to the root BUILD.gn to have it built implicitly.

Change-Id: Iab254fde50f75e2ecd03c3b226883d2fb06c1585
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/154400
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Auto-Submit: Ben Clayton <bclayton@google.com>
Commit-Queue: Antonio Maiorano <amaiorano@google.com>

BUILD.gn

@@ -34,6 +34,9 @@ group("all") {
   if (tint_build_unittests) {
     deps += [ "src/tint/cmd/test:test_cmd" ]
   }
+  if (tint_build_benchmarks) {
+    deps += [ "src/tint/cmd/bench:bench_cmd" ]
+  }
   if (dawn_standalone) {
     deps += [
       "src/dawn/samples",

src/tint/BUILD.gn

@@ -197,10 +197,7 @@ if (tint_build_unittests) {
 if (tint_build_benchmarks) {
   group("google_benchmark") {
     testonly = true
-    public_deps = [
-      "//third_party/google_benchmark",
-      "//third_party/google_benchmark:benchmark_main",
-    ]
+    public_deps = [ "//third_party/google_benchmark" ]
   }
 }