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This adds support for expressions like

    Switch(arg)
      .Case(1, result1)
      .CASES((2, 3), result2)
      .Default(result3)

Currently these expressions are compiled simply as short-hand for

    If(arg == 1, result1)
      .ElseIf(arg == 2 || arg == 3, result2)
      .Else(result3)

but in the future we should be able to optimize it better.

The CASES macro ugliness is unfortunately necessary until we're
allowed to use C++11.  Later we'll be able to change it to
real C++ code like "Cases({2, 3}, result2)" (which clang-format
will then format a bit more nicely too).

BUG=408845
R=jln@chromium.org, jorgelo@chromium.org

Review URL: https://codereview.chromium.org/438683004

Cr-Commit-Position: refs/heads/master@{#293599}