macOS 11.0 SDK cleanup in base/mac/rosetta.mm

Now that we're building against the macOS 11 SDK, do
some cleanup.

Bug: 1105187
Change-Id: I53fe103d8006f03dacdf2a9a2241bf743f39755f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545232
Auto-Submit: Avi Drissman <avi@chromium.org>
Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828443}

base/mac/rosetta.mm

@@ -42,14 +42,14 @@ bool ProcessIsTranslated() {
 #if defined(ARCH_CPU_ARM64)
 
 bool IsRosettaInstalled() {
-  // Chromium currently requires the 10.15 SDK, but code compiled for Arm must
-  // be compiled against at least the 11.0 SDK and will run on at least macOS
-  // 11.0, so this is safe. __builtin_available doesn't work for 11.0 yet;
-  // https://crbug.com/1115294
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunguarded-availability-new"
-  return CFBundleIsArchitectureLoadable(CPU_TYPE_X86_64);
-#pragma clang diagnostic pop
+  if (@available(macOS 11.0, *)) {
+    return CFBundleIsArchitectureLoadable(CPU_TYPE_X86_64);
+  } else {
+    // Arm Macs require at least 11.0 to run, so this branch will never be
+    // taken.
+    NOTREACHED();
+    return false;
+  }
 }
 
 void RequestRosettaInstallation(