Specify correct dependencies for gen_ukm_builders.

Also, make the hash constants use constexpr.

BUG=none

Change-Id: I7abb633d8bfac35eda50f797a2bf88b1ac8f8584
Reviewed-on: https://chromium-review.googlesource.com/811527Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523623}

services/metrics/public/cpp/BUILD.gn

@@ -40,6 +40,12 @@ component("metrics_cpp") {
 action("gen_ukm_builders") {
   script = "//tools/metrics/ukm/gen_builders.py"
 
+  # Re-generate the outputs if the codegen code changes:
+  inputs = [
+    "//tools/metrics/ukm/builders_template.py",
+    "//tools/metrics/ukm/codegen.py",
+    "//tools/metrics/ukm/gen_builders.py",
+  ]
   sources = [
     "//tools/metrics/ukm/ukm.xml",
   ]

tools/metrics/ukm/builders_template.py

@@ -36,14 +36,14 @@ class {event.name} : public ::ukm::internal::UkmEntryBuilderBase {{
   ~{event.name}() override;
 
   static const char kEntryName[];
-  static const uint64_t kEntryNameHash = UINT64_C({event.hash});
+  static constexpr uint64_t kEntryNameHash = UINT64_C({event.hash});
 
 {metric_code}
 }};
 """,
 metric_template="""
   static const char k{metric.name}Name[];
-  static const uint64_t k{metric.name}NameHash = UINT64_C({metric.hash});
+  static constexpr uint64_t k{metric.name}NameHash = UINT64_C({metric.hash});
   {event.name}& Set{metric.name}(int64_t value);
 """)