Revert of Linux UI: Switch to the Gtk3 theme (patchset #3 id:180001 of https://codereview.chromium.org/2670623002/ )

Reason for revert:
Reverting to fix component builds on systems with Mir.
Will reland (probably later today) once https://codereview.chromium.org/2756543002/ lands

Original issue's description:
> Linux UI: Switch to the Gtk3 theme
>
> Sets "use_gtk3 = true" by default.
>
> BUG=79722
> TBR=dpranke@chromium.org
> CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_chromium_msan_rel_ng;master.tryserver.chromium.linux:linux_chromium_dbg_32_ng
>
> > > Review-Url: https://codereview.chromium.org/2670623002
> > > Cr-Commit-Position: refs/heads/master@{#452118}
> > > Committed: https://chromium.googlesource.com/chromium/src/+/872a494bba52e597388aec738d9d681183a3d47b
>
> > Review-Url: https://codereview.chromium.org/2670623002
> > Cr-Commit-Position: refs/heads/master@{#454491}
> > Committed: https://chromium.googlesource.com/chromium/src/+/89be63a39b5014096115d3ec60d13fde39e73283
>
> Review-Url: https://codereview.chromium.org/2670623002
> Cr-Commit-Position: refs/heads/master@{#455596}
> Committed: https://chromium.googlesource.com/chromium/src/+/072522801041708fba94498a1844cb89f24d4590

TBR=erg@chromium.org,mmoss@chromium.org,thomasanderson@google.com
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=79722,700120

Review-Url: https://codereview.chromium.org/2754913002
Cr-Commit-Position: refs/heads/master@{#457516}
4 files changed