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Until C++17 a static constexpr member variable needs a definition.

Without this, kEntryNameHash members and similar metrics name hashes
can't be odr-used (ill-defined).

So now generated UKM builders .cc files will go from (only modified
snippet shown for a sample event):

    const char Identifiability::kEntryName[] = "Identifiability";

to:

    const char Identifiability::kEntryName[] = "Identifiability";
    const uint64_t kEntryNameHash;

And similarly for metrics hashes.

While we are here, this CL cleans up the include header so that it
doesn't include a double //.

I.e. This:

    #include "services/metrics/public/cpp//ukm_builders.h"

Becomes this:

    #include "services/metrics/public/cpp/ukm_builders.h"

Bug: 973801
Change-Id: Icfd234ef953f2cefee5376803be266a2321df020
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2203767Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Commit-Queue: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#770780}