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Currently the exception is not thrown until after serialization.
This makes us consistent with Gecko and WebKit, which throw before serialization.

This fixes the second test case in https://github.com/w3c/web-platform-tests/pull/9672
(not yet pushed to upstream WPT).

Bug: 816447
Change-Id: I30b798b5d21dc8d6f2a40d049ced6aa7f60dd090
Reviewed-on: https://chromium-review.googlesource.com/956254Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542484}