Revert of DevTools: format keys in object previews using sans serif. (patchset #3 id:40001 of https://codereview.chromium.org/2866363003/ )

Reason for revert:
This change broke webkit_tets on multiple builders. See, for example:

https://uberchromegw.corp.google.com/i/chromium.webkit/builders/WebKit%20Linux%20Trusty/builds/26454

Original issue's description:
> DevTools: format keys in object previews using sans serif.
>
> NOTRY=try
>
> Review-Url: https://codereview.chromium.org/2866363003
> Cr-Commit-Position: refs/heads/master@{#471578}
> Committed: https://chromium.googlesource.com/chromium/src/+/14981e159fc950c0a628869e85c166c6c09809ca

TBR=dgozman@chromium.org,pfeldman@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review-Url: https://codereview.chromium.org/2878243002
Cr-Commit-Position: refs/heads/master@{#471584}
25 files changed