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
1 file changed