Gtk: Change NULL to nullptr

This CL changes all instances of NULL to nullptr in libgtkui.

The process was:
$ find chrome/browser/ui/libgtkui -type f | xargs sed
  -i 's/NULL/nullptr/g'
$ git cl format

In addition, remove a workaround that redefines NULL to nullptr since
all references to NULL have been removed.
https://cs.chromium.org/chromium/src/chrome/browser/ui/libgtkui/gtk_ui.cc?rcl=8db1658c62cfe49d1be14bc0186d4c20152cfd84&l=85

BUG=79722
R=erg@chromium.org

Review-Url: https://codereview.chromium.org/2707313002
Cr-Commit-Position: refs/heads/master@{#452198}
24 files changed