SET: Allow an element to be specified multiple times as shared.

This patch allows an element to be specified as shared multiple times.
This was already possible in the API, but we didn't plumb sufficient
information to viz to let it take advantage of it.

Also adds a (manual) test to exercise this behavior.

R=wangxianzhu@chromium.org

Bug: 1174141
Change-Id: I0e7da0889297e96d0ddafa66ae736055a987f5ec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2844997
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#876233}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index f90c424..a0848a9 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -35,6 +35,7 @@
     "benchmarks/unittest_only_benchmark_impl.h",
     "document_transition/document_transition_request.cc",
     "document_transition/document_transition_request.h",
+    "document_transition/document_transition_shared_element_id.cc",
     "document_transition/document_transition_shared_element_id.h",
     "input/actively_scrolling_type.h",
     "input/browser_controls_offset_manager.cc",