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This adds a new value to Download.IOSDownloadMimeType histogram.

From graphics.pixar.com:
USD is the core of Pixar's 3D graphics pipeline, used in every 3D
authoring and rendering application, including Pixar's proprietary
Presto animation system.

USD previews are supported by Safari and logging the usage will help to
decide the priority of supporting USD previews in Chrome.

Bug: 883399
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I40ceac24f21316f2e5681f1f8af9ea538ac7509e
Reviewed-on: https://chromium-review.googlesource.com/1252363Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595483}