PY3 related changes to web tests - 1st iteration

Bug:1197679

Currently, with these changes, it is at a point where tests are picked up and running. But the tests are timing out.
Will address it in the next iteration.
Results on PY3 - https://ci.chromium.org/p/chromium/builders/try/linux-blink-rel/52312?

Change-Id: I2b983b287b2df1d160f6efaadda670764e6a38e7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2787682


Reviewed-by: Dirk Pranke <dpranke@google.com>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Preethi Mohan <preethim@google.com>
Cr-Commit-Position: refs/heads/master@{#872083}