Provide a non_const_ref option to mojom typemap

For a the category of consumers who want
func(T&)
rather than the default
func(const T&)

R=rockot@chromium.org

typemap.
  virtual void SetAccessibilityTree(int32_t cookie, ::ui::AXTreeUpdate& accessibility_tree) = 0;
in
components/printing/common/print.mojom.h
WIthout the change, we have
  virtual void SetAccessibilityTree(int32_t cookie, const ::ui::AXTreeUpdate& accessibility_tree) = 0;

Bug: 332302437
Fixed: 332697604
Test: apply non_const_ref to ui/accessibility/mojom/BUILD.gn, AXTreeUpdate
Change-Id: Ia7e08e1ca6bffd9e5a6e8651fdd9da6b190420b1
Observe: 
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5527832
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1298471}