Document that webview input events are in microseconds.

Bug: b/141992733
Change-Id: Ifff0300df4c36ae8eed67541bda35a5fb0187013
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1869832Reviewed-by: Daniel Nicoara <dnicoara@chromium.org>
Commit-Queue: Kevin Schoedel <kpschoedel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707556}

chromecast/browser/webview/proto/webview.proto

@@ -67,6 +67,7 @@ message MouseEvent {
 message InputEvent {
   int32 event_type = 1;
   int32 flags = 2;
+  // Event timestamps are in monotonically increasing microseconds.
   int64 timestamp = 3;
   KeyInput key = 4;
   TouchInput touch = 5;