Enable OpenH264 assembly on Mac OS

This was disabled in the past due to build failures, which are no longer
reproducible on the latest checkout.

Bug: 600399
Change-Id: I10bbc35d49edf2458d4262fb9201a0997a5e745e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2362974Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Cr-Commit-Position: refs/heads/master@{#799766}

third_party/openh264/BUILD.gn

@@ -40,12 +40,9 @@ config("config") {
   }
 }
 
-# NASM assembly is only checked to be working on Windows and Linux.
-# Mac is known to fail certain tests when building, but actual assembly
-# is believed to work.
 # MSAN builds are flaky with assembler. crbug.com/685168
 
-use_assembler = (is_win || is_linux || is_chromeos) &&
+use_assembler = (is_win || is_mac || is_linux || is_chromeos) &&
                 (current_cpu == "x86" || current_cpu == "x64") && !is_msan
 
 # This IF statement will make the targets visible only on specific builds,