Convert PRESUBMIT.py to python3

Bug: 1212126
Change-Id: I222a407de9afece6ca6dd05d193b396e3dbb53f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2930880
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Dan H <harringtond@chromium.org>
Cr-Commit-Position: refs/heads/master@{#888468}

ui/resources/PRESUBMIT.py

@@ -10,6 +10,8 @@
 for the rules we're checking against here.
 """
 
+USE_PYTHON3 = True
+
 
 def CheckChangeOnUpload(input_api, output_api):
   return _CommonChecks(input_api, output_api)

ui/resources/resource_check/resource_scale_factors.py

@@ -48,7 +48,7 @@ def RunChecks(self):
     def ImageSize(filename):
       with open(filename, 'rb', buffering=0) as f:
         data = f.read(24)
-      if data[:8] != '\x89PNG\r\n\x1A\n' or data[12:16] != 'IHDR':
+      if data[:8] != b'\x89PNG\r\n\x1A\n' or data[12:16] != b'IHDR':
         raise InvalidPNGException
       return struct.unpack('>ii', data[16:24])