Revert of Make the extension's sidebar pane auto-resizable. (patchset #7 id:120001 of https://codereview.chromium.org/1905493002/ )

Reason for revert:
Regression: sources tab extension panels do not render correctly
BUG=622456

Original issue's description:
> Make the extension's sidebar pane auto-resizable.
>
> Setting height of the sidebar pane is a relic from the old design where panes were expandable (https://developer.chrome.com/static/images/devtools-panels.png). In the current design, where panes are tabs, it doesn't make sense.
>
> BUG=438126
>
> Committed: https://crrev.com/7ff544f5fa1d09072def5419d74f8c0e5f42365e
> Cr-Commit-Position: refs/heads/master@{#391965}

TBR=paulirish@chromium.com,pfeldman@chromium.com,paulirish@chromium.org,kdzwinel@gmail.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=438126

Review-Url: https://codereview.chromium.org/2155173003
Cr-Commit-Position: refs/heads/master@{#406149}
6 files changed