The problem popped up when passing the constants by reference
(https://chromium-review.googlesource.com/c/565141).
It's a bit ugly, but, the C++11 standard requires a definition
additionally to the existing declaration in the body of the class:

9.4.2/4: If a static data member is of const literal type, its
  declaration in the class definition can specify a
  brace-or-equal-initializer in which every initializer-clause that is
  an assignment-expression is a constant expression. A static data
  member of literal type can be declared in the class definition with
  the constexpr specifier; if so, its declaration shall specify a
  brace-or-equal-initializer in which every initializer-clause that i
  an assignment-expression is a constant expression. [Note: In both
  these cases, the member may appear in constant expressions. — end
  note] The member shall still be defined in a namespace scope if it is
  odr-used (3.2) in the program and the namespace scope definition shall
  not contain an initializer.

Drive-by: Make the static constants constexpr.

R=bmeurer@chromium.org

Change-Id: Idc3d20bf2adf31d874c23ff8bfec52437789160a
Reviewed-on: https://chromium-review.googlesource.com/567506Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46599}