Log model/usd MIME type downloads.

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}

ios/chrome/browser/download/browser_download_service.h

@@ -38,7 +38,9 @@ enum class DownloadMimeTypeResult {
   VirtualContactFile = 6,
   // text/calendar MIME type.
   iCalendar = 7,
-  kMaxValue = iCalendar,
+  // model/usd MIME type.
+  UniversalSceneDescription = 8,
+  kMaxValue = UniversalSceneDescription,
 };
 
 // Keyed Service which acts as web::DownloadController delegate and routes

ios/chrome/browser/download/browser_download_service.mm

@@ -39,6 +39,9 @@ DownloadMimeTypeResult GetUmaResult(const std::string& mime_type) {
   if (mime_type == "text/calendar")
     return DownloadMimeTypeResult::iCalendar;
 
+  if (mime_type == "model/usd")
+    return DownloadMimeTypeResult::UniversalSceneDescription;
+
   return DownloadMimeTypeResult::Other;
 }
 }  // namespace

tools/metrics/histograms/enums.xml

@@ -12104,6 +12104,7 @@ Called by update_net_error_codes.py.-->
   </int>
   <int value="6" label="Virtual Contact File">text/vcard MIME type.</int>
   <int value="7" label="iCalendar">text/calendar MIME type.</int>
+  <int value="8" label="Universal Scene Description">model/usd MIME type.</int>
 </enum>
 
 <enum name="DownloadNotificationForegroundLifecycle">