Clipboard: Implement ReadPng for Windows

Read PNGs as raw bytes from the clipboard.

PNGs and bitmaps are separate formats on Windows, and for now this only
adds automated testing for reading PNGs through the bitmap format.

Manual tests have verified that this works as-intended on Windows.
To reproduce, follow a version of these instructions modified for
Windows: https://crbug.com/1177229#c1

1) Build a version of Chrome locally with the ReadPng path fully
   implemented and where DataObjectItem::GetAsFile() calls ReadPng.
   See https://crrev.com/c/2961533
2) Download an image which has metadata, such as
   https://github.com/Exiv2/exiv2/blob/old-master/test/data/ReaganSmallPng.png
3) Confirm it has metadata using a clipboard inspector such as
   https://www.metadata2go.com/
4) Navigate to https://evercoder.github.io/clipboard-inspector/
5) Copy image to the system clipboard
6) Paste the image to the browser tab
7) Find the image corresponding to the ".items" property.
   It should be right under the text ".items - 2 items available"
8) Right-click on the image and select
   "Save link as ..." / "Save linked file as ..."
9) Check the image metadata using a clipboard inspector. It should
   match the image which was downloaded in step (2)

Bug: 1201018
Change-Id: I04fa751891ce674016a5ba8f36a99093e5663818
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2861958
Commit-Queue: Austin Sullivan <asully@chromium.org>
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#892396}