heap-profiler: Clean up some comments and C++.

base/trace_event/heap_profiler_allocation_register.h

Index: base/trace_event/heap_profiler_allocation_register.h
diff --git a/base/trace_event/heap_profiler_allocation_register.h b/base/trace_event/heap_profiler_allocation_register.h
index 6d63923f4c164e7a51a2eaf6686d94a87de1f4a9..ac9872f001c218e7e82262d3522ce6c012a2770f 100644
--- a/base/trace_event/heap_profiler_allocation_register.h
+++ b/base/trace_event/heap_profiler_allocation_register.h
@@ -52,7 +52,7 @@ class FixedHashMap {
 
   // Capacity controls how many items this hash map can hold, and largely
   // affects memory footprint.
-  FixedHashMap(size_t capacity)
+  explicit FixedHashMap(size_t capacity)
       : num_cells_(capacity),
         num_inserts_dropped_(0),
         cells_(static_cast<Cell*>(
@@ -252,6 +252,8 @@ class TraceEventMemoryOverhead;
 // freed. Internally it has two hashtables: one for Backtraces and one for
 // actual allocations. Sizes of both hashtables are fixed, and this class
 // allocates (mmaps) only in its constructor.
+//
+// When either hash table hits max size, new inserts are dropped.
 class BASE_EXPORT AllocationRegister {
  public:
   // Details about an allocation.
@@ -366,8 +368,13 @@ class BASE_EXPORT AllocationRegister {
   AllocationMap allocations_;
   BacktraceMap backtraces_;
 
-  // Sentinel used when we run out of backtraces_ storage.
-  BacktraceMap::KVIndex out_of_storage_backtrace_index_;
+  // Sentinel used when the |backtraces_| table is full.
+  //
+  // This is a slightly abstraction to allow for constant propagation. It
+  // knows that the sentinel will be the first item inserted into the table
+  // and that the first index retuned will be 0. The constructor DCHECKs
+  // this assumption.
+  enum : BacktraceMap::KVIndex { kOutOfStorageBacktraceIndex = 0 };
 
   DISALLOW_COPY_AND_ASSIGN(AllocationRegister);
 };