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In jumbo builds many cc files compile in the same translation
unit and then they share the same anonymous namespace. Having
the same constant several times will then result in compilation
failures.

In preparation for jumbo in chrome/browser, this patch
deduplicates the kMHTMLExtension constant and puts it in the
utility class.

An alternative would be to use the literal string "mhtml" without
going through a constant at all.

Bug: 746957
Change-Id: I869f15c17bcf14a633a3e0711c7e469159e3083f
Reviewed-on: https://chromium-review.googlesource.com/897944Reviewed-by: Cathy Li <chili@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#533764}