cmd/trace: update HTML; expand viewer to whole window
cmd/trace: update HTML; expand viewer to whole window

This updates the HTML served for the trace viewer to follow the latest
revision of the example from the upstream tracing project.

The main thing this adds is CSS for the trace viewer (which was
actually in the example at the originally referenced revision, so I'm
not sure why it got dropped). In particular, this expands the trace
viewer to use the entire browser client area, which fixes several
problems with the current page:

1. The details pane gets cut off at a strange place and can get a
scroll bar even if there's plenty of room below it on the page. This
fixes the bottom of the details pane to the bottom of the window.

2. If the track view is very tall (lots of procs), there's no way to
view the top tracks and the details pane at the same time. This fixes
this problem by limiting the height of the track view to something
less than the height of the window so it gets a scroll bar of its own
if necessary.

3. Dragging the divider between the track pane and the details pane
actually moves the bottom of the details pane without moving the
divider. Fixing the height of the trace viewer fixes this problem.

Change-Id: Ia811e72a7413417ca21c45e932c9db2724974633
Reviewed-on: https://go-review.googlesource.com/54632
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>

josharian/go,Cofyc/go,polyverse-security/go,cloudflare/go,CAFxX/go,codestation/go,golang/go,josharian/go,akutz/go,Cofyc/go,golang/go,golang/go,cloudflare/go,codestation/go,CAFxX/go,polyverse-security/go,cloudflare/go,nawawi/go,nawawi/go,akutz/go,golang/go,cloudflare/go,Cofyc/go,golang/go,CAFxX/go,nawawi/go,CAFxX/go,nawawi/go,polyverse-security/go,codestation/go,cloudflare/go,Cofyc/go,akutz/go,nawawi/go,Cofyc/go,polyverse-security/go,codestation/go,CAFxX/go,josharian/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,nawawi/go,Cofyc/go,codestation/go,akutz/go,golang/go,golang/go,polyverse-security/go,polyverse-security/go,josharian/go,codestation/go,codestation/go,Cofyc/go,christopher-henderson/Go,golang/go,nawawi/go,CAFxX/go,CAFxX/go,nawawi/go,codestation/go,CAFxX/go,josharian/go,akutz/go,josharian/go,josharian/go,polyverse-security/go,josharian/go,cloudflare/go,polyverse-security/go,Cofyc/go,akutz/go