Fix cluster-fuzz crash with internal EventSender API.
Fix cluster-fuzz crash with internal EventSender API.

In release mode a DCHECK was compiled out and caused a crash; it was
correctly asserting in the Debug builds. Change the DCHECK into a
runtime check that throws an exception. This addresses the cluster fuzz
crash.

BUG=472058
TEST=out/Release/content_shell --dump-render-tree --use-gl=any --disable-gl-drawing-for-test file:///work/issues/472058/test.html

Review URL: https://codereview.chromium.org/1140813007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#330659}

Just-D/chromium-1,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,Just-D/chromium-1,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,Chilledheart/chromium,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Just-D/chromium-1,Chilledheart/chromium,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,hgl888/chromium-crosswalk