Require the 10.15 SDK to build Chromium.

BUG=1023913

Change-Id: I11d24c83ca1f8f99fc64c612c383fd3979e00551
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036406Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#742252}

build/config/mac/mac_sdk_overrides.gni

@@ -9,7 +9,7 @@ _sdk_min_from_env = getenv("FORCE_MAC_SDK_MIN")
 declare_args() {
   # Minimum supported version of the Mac SDK.
   if (_sdk_min_from_env == "") {
-    mac_sdk_min = "10.14"
+    mac_sdk_min = "10.15"
   } else {
     mac_sdk_min = _sdk_min_from_env
   }