[ukm-builders] constexprs need definitions.

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}

tools/metrics/ukm/builders_template.py

@@ -48,13 +48,12 @@ metric_template="""
   {event.name}& Set{metric.name}(int64_t value);
 """)
 
-IMPL = codegen.Template(
-basename="ukm_builders.cc",
-file_template="""
+IMPL = codegen.Template(basename="ukm_builders.cc",
+                        file_template="""
 // Generated from gen_builders.py.  DO NOT EDIT!
 // source: ukm.xml
 
-#include "{file.dir_path}/ukm_builders.h"
+#include "{file.dir_path}ukm_builders.h"
 
 namespace ukm {{
 namespace builders {{
@@ -64,8 +63,9 @@ namespace builders {{
 }}  // namespace builders
 }}  // namespace ukm
 """,
-event_template="""
+                        event_template="""
 const char {event.name}::kEntryName[] = "{event.raw_name}";
+const uint64_t {event.name}::kEntryNameHash;
 
 {event.name}::{event.name}(ukm::SourceId source_id) :
   ::ukm::internal::UkmEntryBuilderBase(source_id, kEntryNameHash) {{
@@ -79,8 +79,9 @@ const char {event.name}::kEntryName[] = "{event.raw_name}";
 
 {metric_code}
 """,
-metric_template="""
+                        metric_template="""
 const char {event.name}::k{metric.name}Name[] = "{metric.raw_name}";
+const uint64_t {event.name}::k{metric.name}NameHash;
 
 {event.name}& {event.name}::Set{metric.name}(int64_t value) {{
   SetMetricInternal(k{metric.name}NameHash, value);

tools/metrics/ukm/gen_builders_test.py

@@ -71,6 +71,7 @@ class {name} final : public ::ukm::internal::UkmEntryBuilderBase {{
     self.assertIn(
         """
 const char {name}::kEntryName[] = "{rawName}";
+const uint64_t {name}::kEntryNameHash;
 
 {name}::{name}(ukm::SourceId source_id) :
   ::ukm::internal::UkmEntryBuilderBase(source_id, kEntryNameHash) {{
@@ -84,6 +85,7 @@ const char {name}::kEntryName[] = "{rawName}";
     self.assertIn(
         """
 const char {eventName}::k{metricName}Name[] = "{metricRawName}";
+const uint64_t {eventName}::k{metricName}NameHash;
 
 {eventName}& {eventName}::Set{metricName}(int64_t value) {{
   SetMetricInternal(k{metricName}NameHash, value);