[release-branch.go1.13] cmd/trace: update to use WebComponents V0 polyfill
    
    Old trace viewer stopped working with Chrome M80+ because the
    old trace viewer heavily depended on WebComponents V0 which are deprecated.
    Trace viewer recently migrated to use WebComponents V0 polyfill
    (crbug.com/1036492). This CL brings in the newly updated trace_viewer_full.html
    (sync'd @ 9508452e)
    and updates the javascript snippet included in the /trace endpoint
    to use the polyfill.
    
    This brings in webcomponents.min.js copied from
    https://chromium.googlesource.com/catapult/+/9508452e18f130c98499cb4c4f1e1efaedee8962/third_party/polymer/components/webcomponentsjs/webcomponents.min.js
    
    That is necessary because the /trace endpoint needs to import
    the vulcanized trace_viewer_full.html.
    
    It's possible that some features are not working correctly with
    this polyfill. In that case, report the issue to crbug.com/1036492.
    There will be a warning message in the UI (yellow banner above the timeline)
    which can be hidden by clicking the 'hide' button.
    
    This allows to render the trace in browsers other than chrome in theory,
    but I observed some buttons and functions still don't work outside
    chrome.
    
    Updates #34374.
    Fixes #37342.
    
    Change-Id: Ib575f756f5e6b22ad904ede6e4d224a995ebe259
    Reviewed-on: https://go-review.googlesource.com/c/go/+/219997
    Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
    TryBot-Result: Gobot Gobot <gobot@golang.org>
    Reviewed-by: Austin Clements <austin@google.com>
    (cherry picked from commit 75ea964b3f6073076e1a86a0de2be9a2f159da24)
    Reviewed-on: https://go-review.googlesource.com/c/go/+/220321
    Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>