map view is off-center

[Impact]
Some widgets such as the map view in gnome-maps 3.18.3 are off-center. (The upstream bug was originally about polari 3.19, an IRC app from GNOME.)

Screenshot attached.

[Test Case]
1. Install gnome-maps 3.18.3-0ubuntu1 (as of today, in xenial-proposed)
2. Open the maps app. Is there a blank area on one side of the map view?
3. Install the updated gtk-3 packages
4. Open the maps app. The map view should fill the map view space.

[Regression Potential]
The minimal fix has been shipped with GTK since 3.19.10. (Therefore, this bug is already fixed in yakkety-proposed.)

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: libgtk-3-0 3.18.9-1ubuntu3.1
ProcVersionSignature: Ubuntu 4.4.0-31.50-generic 4.4.13
Uname: Linux 4.4.0-31-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.1
Architecture: amd64
CurrentDesktop: GNOME
Date: Tue Aug 2 14:58:11 2016
InstallationDate: Installed on 2016-07-31 (2 days ago)
InstallationMedia: Ubuntu-GNOME 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160720)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: gtk+3.0
UpgradeStatus: No upgrade log present (probably fresh install)