ios: mark deprecation warning silence with crbug

TBR=rohitrao@chromium.org

Bug: 933827
Change-Id: I9c4b3e2f8f1088cb5a2c78a6aed500a682874edd
Reviewed-on: https://chromium-review.googlesource.com/c/1478488Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#633784}

ios/chrome/test/google_toolbox_unittest.mm

@@ -15,6 +15,7 @@
 
 using GoogleToolboxForMacTest = PlatformTest;
 
+// TODO(crbug.com/933827): Remove this pragma.
 #pragma clang diagnostic push
 #pragma clang diagnostic ignored "-Wdeprecated-declarations"