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This is in preparation for making more classes use the lookup trie.

Difference before and after for HTMLElementLookupTrie.cpp is here:
https://www.diffchecker.com/lqhiueko

BUG=606695

Review-Url: https://codereview.chromium.org/1978703002
Cr-Commit-Position: refs/heads/master@{#393478}