Specify Xcode 9.2 in dimensions.

This fixes an issue in
https://skia-review.googlesource.com/c/skcms/+/145733, which assumes
Xcode 9.2 is available without limiting to bots where it is installed.

Change-Id: I1b7db49cbcddf90f216adea6aed5340fca25e7a7
Reviewed-on: https://skia-review.googlesource.com/145738
Commit-Queue: Mike Klein <mtklein@google.com>
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/infra/bots/gen_tasks.go b/infra/bots/gen_tasks.go
index 812c897..b2f6129 100644
--- a/infra/bots/gen_tasks.go
+++ b/infra/bots/gen_tasks.go
@@ -20,7 +20,7 @@
 	dimensions := map[string][]string{
 		// For the moment we'd rather not run bots on Skylakes, which support AVX-512.
 		"skcms-Linux": []string{"os:Linux", "cpu:x86-64-Haswell_GCE"},
-		"skcms-Mac":   []string{"os:Mac"},
+		"skcms-Mac":   []string{"os:Mac", "xcode_version:9.2"},
 		// We think there's something amiss building on Win7 or Win8 bots, so restrict to 2016.
 		"skcms-Win": []string{"os:Windows-2016Server"},
 	}
diff --git a/infra/bots/tasks.json b/infra/bots/tasks.json
index 8295b6f..c8ea4ec 100644
--- a/infra/bots/tasks.json
+++ b/infra/bots/tasks.json
@@ -79,6 +79,7 @@
       ],
       "dimensions": [
         "os:Mac",
+        "xcode_version:9.2",
         "gpu:none",
         "pool:Skia"
       ],