Fix GPUTiming offset so it is stable within significant digits.

ui/gl/gpu_timing.cc

 // Copyright (c) 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "ui/gl/gpu_timing.h"
 
 #include "base/macros.h"
 #include "base/time/time.h"
 #include "ui/gl/gl_bindings.h"
 #include "ui/gl/gl_context.h"
@@ skipping 324 matching lines @@
   }
   return disjoint_counter_;
 }
 
 int64_t GPUTimingImpl::CalculateTimerOffset() {
   if (!offset_valid_) {
     if (timer_type_ == GPUTiming::kTimerTypeDisjoint ||
         timer_type_ == GPUTiming::kTimerTypeARB) {
       GLint64 gl_now = 0;
       glGetInteger64v(GL_TIMESTAMP, &gl_now);
-      int64_t micro_now = NanoToMicro(gl_now);
-      offset_ = GetCurrentCPUTime() - micro_now;
-      offset_valid_ = (timer_type_ == GPUTiming::kTimerTypeARB);
+      const int64_t cpu_time = GetCurrentCPUTime();
+      const int64_t micro_offset = cpu_time - NanoToMicro(gl_now);
+
+      // We cannot expect these instructions to run with the accuracy
+      // within 1 microsecond, instead discard differences which are less
+      // than a single millisecond.
+      base::TimeDelta delta =
+          base::TimeDelta::FromMicroseconds(micro_offset - offset_);
+
+      if (delta.magnitude().InMilliseconds() >= 1) {
+        offset_ = micro_offset;
+        offset_valid_ = (timer_type_ == GPUTiming::kTimerTypeARB);
+      }
     } else {
       offset_ = 0;
       offset_valid_ = true;
     }
   }
   return offset_;
 }
 
 scoped_refptr<QueryResult> GPUTimingImpl::BeginElapsedTimeQuery() {
   DCHECK(timer_type_ != GPUTiming::kTimerTypeInvalid);
@@ skipping 267 matching lines @@
 
 void GPUTimingClient::ForceTimeElapsedQuery() {
   DCHECK(gpu_timing_);
   gpu_timing_->ForceTimeElapsedQuery();
 }
 
 GPUTimingClient::~GPUTimingClient() {
 }
 
 }  // namespace gfx