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Remove BUILD dependencies for merge_xml action and list all xmls that
need to be merged in merge_xml.py instead because listing them in the
sources is too verbose as we split up smaller xmls.

Bug: 1116096
Change-Id: I1ba1e09feece3442a673d4ba41868f36e1135298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2359231
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798647}