Don't report array buffer allocations less than 1 Mb.

History has shown that 99.93% (or more) of all memory allocations are less
than 1 megabyte, and they all appear in the same UMA stat entry.

To give perspective, the entry for <= 1Mb is about 20,000 times larger
than any other entry in the table. This makes the distribution in the
table hard to see.

And, for allocation failures at this size, the percentage of failures
(when compared to number of requests) is soo small (millions to one)
that little data can be gleamed from the <= 1Mb entry.

Note: requires CL https://codereview.chromium.org/2867483002

BUG=chrome:704922
R=bradnelson@chromium.org, bbudge@chromium.org, isherman@chromium.org

Review-Url: https://codereview.chromium.org/2856663002
Cr-Commit-Position: refs/heads/master@{#45148}

src/counters.h

@@ -939,7 +939,8 @@ class RuntimeCallTimerScope {
      51)                                                                      \
   HR(wasm_functions_per_wasm_module, V8.WasmFunctionsPerModule.wasm, 1,       \
      100000, 51)                                                              \
-  HR(array_buffer_big_allocations, V8.ArrayBufferBigAllocations, 0, 4096, 13) \
+  HR(array_buffer_big_allocations, V8.ArrayBufferLargeAllocations, 0, 4096,   \
+     13)                                                                      \
   HR(array_buffer_new_size_failures, V8.ArrayBufferNewSizeFailures, 0, 4096, 13)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                               \

src/objects.cc

@@ -19560,8 +19560,9 @@ bool JSArrayBuffer::SetupAllocatingData(Handle<JSArrayBuffer> array_buffer,
   // Prevent creating array buffers when serializing.
   DCHECK(!isolate->serializer_enabled());
   if (allocated_length != 0) {
-    isolate->counters()->array_buffer_big_allocations()->AddSample(
-        ConvertToMb(allocated_length));
+    if (allocated_length >= MB)
+      isolate->counters()->array_buffer_big_allocations()->AddSample(
+          ConvertToMb(allocated_length));
     if (initialize) {
       data = isolate->array_buffer_allocator()->Allocate(allocated_length);
     } else {