Use 10.15 Mac builders

This is necessary to use the latest Xcode.

Change-Id: I57e15d29ade3920a1d5734f8ec47a52eea49a97d
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/10260
Reviewed-by: Scott Graham <scottmg@chromium.org>

cr-buildbucket.cfg

diff --git a/cr-buildbucket.cfg b/cr-buildbucket.cfg
index 43ca908..c9787b3 100644
--- a/cr-buildbucket.cfg
+++ b/cr-buildbucket.cfg
@@ -78,7 +78,7 @@
 
     builders {
       name: "mac"
-      dimensions: "os:Mac-10.13"
+      dimensions: "os:Mac-10.15"
       dimensions: "cpu:x86-64"
       mixins: "mac_caches"
     }
@@ -120,7 +120,7 @@
 
     builders {
       name: "mac"
-      dimensions: "os:Mac-10.13"
+      dimensions: "os:Mac-10.15"
       dimensions: "cpu:x86-64"
       mixins: "mac_caches"
     }