Remove fixed header at chrome://inspect

Remove fixed header at chrome://inspect

The fixed header gets in the way when the user zooms in and attempts to
scroll the page. To fix this issue, the header is no longer at a
viewport-fixed position, but at a static position within the main
content. The location specified by the page header is already displayed
in the left navigation bar, so this change does not hurt usability.

BUG=492124
Review URL: https://codereview.chromium.org/1187813004

Cr-Commit-Position: refs/heads/master@{#334722}
(cherry picked from commit 0d96227ee3378a3b0676f7923560c69f0a8209c7)

Committed: https://chromium.googlesource.com/chromium/src/+/56cc9bd9cd4b010789298c8200170b06dd599c54

[robwu, 2015-06-17 22:36:13]

Committed patchset #1 (id:1) manually as
56cc9bd9cd4b010789298c8200170b06dd599c54.