Speculative fix for 0-valued performance.memory

This CL ensures that the first call to MaybeUpdate() results in calling
Update() so that MemoryInfo is populated. Technically the monotonic
clock could be initialized to a small value and in this case the first
time this check occurs it could fail, whereas we want it to pass:
"now - last_update_time_ >= delta_allowed"

Bug: 883490
Change-Id: I7143d453709db38068919cff9585c50e06e789ac
Reviewed-on: https://chromium-review.googlesource.com/1228886Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Nicolás Peña Moreno <npm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592071}

third_party/blink/renderer/core/timing/memory_info.cc

@@ -81,7 +81,8 @@ class HeapSizeCache {
     TimeDelta delta_allowed = precision == MemoryInfo::Precision::Bucketized
                                   ? kTwentyMinutes
                                   : kFiftyMs;
-    if (now - last_update_time_ >= delta_allowed) {
+    if (!last_update_time_.has_value() ||
+        now - last_update_time_.value() >= delta_allowed) {
       Update(precision);
       last_update_time_ = now;
     }
@@ -97,7 +98,7 @@ class HeapSizeCache {
     info_.js_heap_size_limit = QuantizeMemorySize(info_.js_heap_size_limit);
   }
 
-  TimeTicks last_update_time_;
+  base::Optional<TimeTicks> last_update_time_;
 
   HeapInfo info_;
   DISALLOW_COPY_AND_ASSIGN(HeapSizeCache);
@@ -164,6 +165,9 @@ MemoryInfo::MemoryInfo(Precision precision) {
     GetHeapSize(info_);
   else
     HeapSizeCache::ForCurrentThread().GetCachedHeapSize(info_, precision);
+  // The values must have been computed, so totalJSHeapSize must be greater than
+  // 0.
+  DCHECK_GT(totalJSHeapSize(), 0u);
 }
 
 }  // namespace blink

third_party/blink/renderer/core/timing/memory_info_test.cc

@@ -231,4 +231,24 @@ TEST_F(MemoryInfoTest, FlagEnabled) {
             precise_memory->usedJSHeapSize());
 }
 
+TEST_F(MemoryInfoTest, ZeroTime) {
+  // In this test, we make sure that even if the CurrentTimeTicks() value is
+  // very close to 0, we still obtain memory information from the first call to
+  // MemoryInfo::Create. We cannot just subtract CurrentTimeTicks() here
+  // because many places have DCHECKs for !time.is_null(), which would be hit if
+  // we set the clock to be exactly 0.
+  AdvanceClock(-CurrentTimeTicksInSeconds() + 0.0001);
+  V8TestingScope scope;
+  v8::Isolate* isolate = scope.GetIsolate();
+  std::vector<v8::Local<v8::ArrayBuffer>> objects;
+  objects.push_back(v8::ArrayBuffer::New(isolate, 100));
+
+  MemoryInfo* precise_memory =
+      MemoryInfo::Create(MemoryInfo::Precision::Precise);
+  CheckValues(precise_memory, MemoryInfo::Precision::Precise);
+  EXPECT_LT(0u, precise_memory->usedJSHeapSize());
+  EXPECT_LT(0u, precise_memory->totalJSHeapSize());
+  EXPECT_LT(0u, precise_memory->jsHeapSizeLimit());
+}
+
 }  // namespace blink