[infra] Ensure Mac xSAN backfill tasks can find Xcode.

This CL prepares for installing Xcode in a Swarming cache dir from a
CIPD package. https://skia-review.googlesource.com/c/skia/+/144608

After that, I hope to isolate the xSAN libs in the Build task so that
we don't need Xcode on the xSAN test bots. Baby steps.

Change-Id: I61dbe9474eceff42cbfd824c429db382b68a8c35
Reviewed-on: https://skia-review.googlesource.com/145143
Auto-Submit: Ben Wagner <benjaminwagner@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
Commit-Queue: Ben Wagner <benjaminwagner@google.com>

infra/bots/recipe_modules/flavor/default.py

@@ -206,6 +206,11 @@ class DefaultFlavor(object):
       ld_library_path.append(clang_linux + '/msan')
 
     if any('SAN' in t for t in extra_tokens):
+      if self.m.vars.builder_cfg.get('os', '') == 'Mac':
+        # TODO(dogben): Remove after isolating xSAN libraries in Build tasks.
+        self.m.step('select xcode', [
+            'sudo', 'xcode-select', '-switch', '/Applications/Xcode9.2.app'])
+
       # Sanitized binaries may want to run clang_linux/bin/llvm-symbolizer.
       path.append(clang_linux + '/bin')
       # We find that testing sanitizer builds with libc++ uncovers more issues

infra/bots/recipe_modules/flavor/examples/full.expected/Test-Mac-Clang-MacMini7.1-CPU-AVX-x86_64-Debug-All-ASAN.json

@@ -139,6 +139,15 @@
     "infra_step": true,
     "name": "write SVG_VERSION"
   },
+  {
+    "cmd": [
+      "sudo",
+      "xcode-select",
+      "-switch",
+      "/Applications/Xcode9.2.app"
+    ],
+    "name": "select xcode"
+  },
   {
     "cmd": [
       "[START_DIR]/build/dm",