[Sensors] Improvements in shared buffer managing

device/generic_sensor/public/interfaces/sensor_provider.mojom

 // Copyright 2016 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.
 
 module device.mojom;
 
 import "sensor.mojom";
 
 struct SensorInitParams {
   // The shared memory handle used to fetch the sensor reading.
   handle<shared_buffer> memory;
 
   // The offset at which shared buffer must be mapped.
   uint64 buffer_offset;
 
   // The ReportingMode supported by the sensor.
   ReportingMode mode;
 
   // Default sensor configuration.
   SensorConfiguration default_configuration;
 
-  // Note: Each sensor's read buffer contains 4 tightly packed 64-bit floating
-  // point fields, its layout is: { double timestamp; double values[3] }.
-  // So it has a fixed size 4*8 = 32 bytes.
-  const uint64 kReadBufferSize = 32;
+  // Each sensor's read buffer contains 4 tightly packed 64-bit floating
+  // point fields (please see sensor_reading.h) and a seqlock, so its size is
+  // 5 * 8 = 40 bytes.
+  const uint64 kReadBufferSizeForTests = 40;
 };
 
 interface SensorProvider {
   // Gets the sensor interface by the given type.
   //
   // |type| type of the sensor.
   //
   // |sensor_request| the Sensor interface instance to be initialized.
   //
   // |init_params| on success will contain the SensorInitParams describing the
   //               sensor details,
   //               contains null on failure.
   // |client_request| on success contains a request to be bound by the client,
   //                  contains null on failure.
   GetSensor(SensorType type, Sensor& sensor_request) => (
       SensorInitParams? init_params,
       SensorClient&? client_request);
 };