Do not override shadow elevation for drag window widget

Two drag windows are created for each overview item: one for the
OverviewItemWidget and another within the overview
(crrev.com/c/5925082). That CL also introduced an option in
`DragWindowController` to control drag window shadows.

For widgets, shadows can be skipped by setting
`views::Widget::InitParams::ShadowType::kNone`, which internally sets
the shadow elevation to `wm::kShadowElevationNone` [1], signaling
ShadowController to not create a shadow.

However, `DragWindowDetails::CreateDragWindow()` overrides this setting
after widget creation, unconditionally creating a shadow. This extra,
shadow results in a visual artifact.

[1]
https://source.chromium.org/chromium/chromium/src/+/main:ui/views/widget/native_widget_aura.cc;l=192-196;drc=b6620a02fa498df5297e53241b54a31f488ca440

Bug: b:372909320, b:373615844
Change-Id: I0e1fee0ae80fd2f73a3c4da806684c59c048a4b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6486270
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Commit-Queue: Zoraiz Naeem <zoraiznaeem@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1453493}