linux_aura: Call MapThemeImage for resources missing in native theme

For resources not provided by the native theme (i.e. GTK2UI), fall
back to desktop (not ash) resources by calling MapThemeImage in this
case.

BUG=383711
R=erg@chromium.org
TEST=The shadow running along the top edge of the address bar to the
right of the tabs is visible again, especially when using a bright GTK
theme like Adwaita or Radiance. (see crbug.com/383711)

Review URL: https://codereview.chromium.org/1243513002

Cr-Commit-Position: refs/heads/master@{#339744}

AUTHORS

@@ -326,6 +326,7 @@ Lauren Yeun Kim <lauren.yeun.kim@gmail.com>
 Lauri Oherd <lauri.oherd@gmail.com>
 Legend Lee <guanxian.li@intel.com>
 Leith Bade <leith@leithalweapon.geek.nz>
+Leo Wolf <jclw@ymail.com>
 Leon Han <leon.han@intel.com>
 Leung Wing Chung <lwchkg@gmail.com>
 Li Yin <li.yin@intel.com>

chrome/browser/ui/views/frame/browser_desktop_window_tree_host_x11.cc

@@ -28,7 +28,7 @@ class DesktopThemeProvider : public ui::ThemeProvider {
     return delegate_->UsingSystemTheme();
   }
   gfx::ImageSkia* GetImageSkiaNamed(int id) const override {
-    if (delegate_->UsingSystemTheme())
+    if (delegate_->UsingSystemTheme() && delegate_->HasCustomImage(id))
       return delegate_->GetImageSkiaNamed(id);
 
     return delegate_->GetImageSkiaNamed(