Fix memory leak in NavButtonProviderGtk3

Gtk increases the reference count of the cairo_pattern_t before returning it to
clients, so this CL adds the appropriate call to cairo_pattern_destroy().

R=thestig

Change-Id: I540c635e15d086aa515c0b1bc3953c1008df5607
Reviewed-on: https://chromium-review.googlesource.com/1058031Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558414}

chrome/browser/ui/libgtkui/nav_button_provider_gtk3.cc

@@ -233,6 +233,7 @@ class NavButtonImageSource : public gfx::ImageSkiaSource {
       ApplyCssToContext(button_context,
                         ".titlebutton { background-size: contain; }");
     }
+    cairo_pattern_destroy(cr_pattern);
 
     // Gtk doesn't support fractional scale factors, but chrome does.
     // Rendering the button background and border at a fractional