Support the py3 value for 'sys.platform' in scripts/sizes.py

After https://chromium-review.googlesource.com/c/chromium/src/+/3301364,
executions of this script on the bots are failing with:
Unsupported sys.platform 'linux'.

eg: https://ci.chromium.org/ui/p/chrome/builders/ci/linux-chrome/39349/overview

Likely because of the value of sys.platform changed from py2 to py3.
Supporting both vals should avoid the error.

Bug: 1274788
Change-Id: Ibd3d463bcaee4ae390a224ff8cde14f00a86124a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3307182
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/main@{#946195}

infra/scripts/sizes.py

@@ -448,7 +448,7 @@ def main():
     default_platform = 'win'
   elif sys.platform.startswith('darwin'):
     default_platform = 'mac'
-  elif sys.platform == 'linux2':
+  elif sys.platform.startswith('linux'):
     default_platform = 'linux'
   else:
     default_platform = None