Disable use of deprecated warnings for material_components_ios

Material components marks some of its API as deprecated but
still use them internally and recommends not treating those
warning as error [1].
Disable the warnings when compiling material_components_ios.
[1]: https://github.com/material-components/material-components-ios/tree/develop/howto/build-env

Bug: 823767
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I377c971130372e80109e54cd29dd21def29067d2
Reviewed-on: https://chromium-review.googlesource.com/973002
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544680}

ios/third_party/material_components_ios/BUILD.gn

@@ -31,6 +31,13 @@ config("config") {
   ]
 }
 
+config("disable_deprecated_errors") {
+  cflags = [
+    "-Wno-deprecated",
+    "-Wno-deprecated-declarations",
+  ]
+}
+
 _icon_names = [
   "ic_check",
   "ic_check_circle",
@@ -322,6 +329,10 @@ source_set("material_components_ios") {
     ":config",
     "//build/config/compiler:enable_arc",
     "//build/config/compiler:no_chromium_code",
+
+    # material_components_ios internally uses deprecated methods.
+    # Disable the warning about deprecation usage.
+    ":disable_deprecated_errors",
   ]
 }