Running Chrome browser on Linux with the Gtk theme was failing on a NOTREACHED in Gtk2UI::GetTint() for the ThemeProperties::COLOR_BACKGROUND_TAB id that was added here: https://codereview.chromium.org/1592553002. The cross-platform default is sufficient so Gtk2UI::GetTint() was updated to handle the ThemeProperties::COLOR_BACKGROUND_TAB value.
Running Chrome browser on Linux with the Gtk theme was failing
on a NOTREACHED in Gtk2UI::GetTint() for the
ThemeProperties::COLOR_BACKGROUND_TAB id that was added here:
https://codereview.chromium.org/1592553002. The cross-platform
default is sufficient so Gtk2UI::GetTint() was updated to handle
the ThemeProperties::COLOR_BACKGROUND_TAB value.

BUG=583662

TEST=Manual

Review URL: https://codereview.chromium.org/1661793002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#374386}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk