Versions of GTK3 up to 3.14.7 had a bug that would cause a segfault
when a GtkFileChooser was open and a hidden file in the directory was
deleted [1].

Chrome can trigger this bug in the following scenario:
1. Open 2 windows
2. Open a save dialog in each window
3. Close one of the dialogs

Chrome saves a file in two stages: it writes the contents to a hidden
file, and then renames it to the actual file.  This is so the file can
be downloading as the user is choosing a file to save to.

By default, the hidden file is placed into whichever directory the
save dialog opens with.  In the case of 2 save dialogs open at once,
if the user saves the file (renames the hidden file) or cancels the
download (deletes the hidden file), the GTK bug would be triggered.

This CL changes the default directory for the hidden files to
XDG_DATA_HOME in an effort to make triggering the GTK bug far less
likely.

[1] https://mail.gnome.org/archives/commits-list/2014-December/msg02447.html

BUG=728501
R=thestig@chromium.org
TBR=qinmin@chromium.org

Review-Url: https://codereview.chromium.org/2924433002
Cr-Commit-Position: refs/heads/master@{#479213}