Revert "Require the 10.15 SDK to build Chromium."

This reverts commit 50cf1bfe.

Reason for revert: give people time to update

Original change's description:
> Require the 10.15 SDK to build Chromium.
> 
> BUG=1023913
> 
> Change-Id: Ibd15e2c76413639c0caf8d1d255add07a815bcdf
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2033986
> Auto-Submit: Avi Drissman <avi@chromium.org>
> Reviewed-by: Robert Sesek <rsesek@chromium.org>
> Commit-Queue: Avi Drissman <avi@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#737904}

TBR=avi@chromium.org,rsesek@chromium.org

Change-Id: I38a384dcec4cfca06a4de27e20a9c52d78e5dfea
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 1023913
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2036378Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737918}

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 = "10.15"
+    mac_sdk_min = "10.14"
   } else {
     mac_sdk_min = _sdk_min_from_env
   }