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New Clang versions started taking enumerators into account for the
-Wshadow warning:

../../ui/compositor/layer_animation_element.h:41:5: warning: declaration
shadows a variable in namespace 'ui' [-Wshadow]
    UNKNOWN = 0,
    ^
../../ui/latency/latency_info.h:97:3: note: previous declaration is here
  UNKNOWN,
  ^

By making ui::SourceEventType an enum class, we avoid putting the enumerators
in such a large namespace as ui.

TBR=jochen

Bug: 895475
Change-Id: I99347d754cb21dacb5d7c4bae975d3310dcb19fd
Reviewed-on: https://chromium-review.googlesource.com/c/1284951
Commit-Queue: Hans Wennborg <hans@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600177}