Simplify DoParseProgram

DoParseProgram doesn't appear to need to receive toplevel scopes as
arguments; it can properly set the end_position of the scopes to the
scanner's position after parsing is complete.

R=marja@chromium.org
BUG=
LOG=N

Review URL: https://codereview.chromium.org/1091743002

Cr-Commit-Position: refs/heads/master@{#27880}