Make swift-format default True on Mac hosts

Also make swift lint strict to catch every formatting errors.

Bug: 1263872
Change-Id: Ie717c1f03ec1a0257cafceecc2055767d0be2077
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3605589
Reviewed-by: Brian Ryner <bryner@google.com>
Auto-Submit: Olivier Robin <olivierrobin@chromium.org>
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
diff --git a/swift_format.py b/swift_format.py
index 50536fe..219211d 100644
--- a/swift_format.py
+++ b/swift_format.py
@@ -40,6 +40,8 @@
 
 
 def IsSwiftFormatSupported():
+  if sys.platform != 'darwin':
+    return False
   try:
     FindSwiftFormatToolInChromiumTree()
     return True