Remove Google Storage scheme prefix correctly from gs base name in webpages_playback

In python, str.lstrip([chars]) means "lstrip any of the chars from str",
not "remove prefix". Thus "gs://skia".lstrip("gs://") yields "kia".

Replace with custom function stripping the prefix from lstrip()ed gs
base name.

Review URL: https://codereview.chromium.org/780493002
Update webpages_playback documentation

Update the documentation on how to run webpages_playback. It was out of
date.

Review URL: https://codereview.chromium.org/778633002
Make debugger not fail with webpages_playback script and multiple skp files

The debugger accepts either a single skp or a directory as a parameter.
The webpages_playback script used to pass multiple files using a '*'
glob.

Review URL: https://codereview.chromium.org/775073002
First step to moving vertex attributes to the geometryProcessor

BUG=skia:

Review URL: https://codereview.chromium.org/761563002