LaCrOS: Replace defined(OS_CHROMEOS) with BUILDFLAG(IS_CHROMEOS_ASH)

The change is mostly mechanical replacing defined(OS_CHROMEOS) with
BUILDFLAG(IS_CHROMEOS_ASH) and GN variable is_chromeos with is_ash
with some special cases (For those cases please refer to
http://go/lacros-macros).

The patch is made in preparation to switching lacros build from
target_os=linux to target_os=chromeos. This will prevent lacros from
changing behaviour after the switch.

Bug: 1052397
Change-Id: I34cfe1a8d474a2b146138ce2502c4cb1875c37cb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2494194
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Yuta Hijikata <ythjkt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#825694}

diff --git a/cc/raster/gpu_raster_buffer_provider.cc b/cc/raster/gpu_raster_buffer_provider.cc
index 3d106918..76ecd0d 100644
--- a/cc/raster/gpu_raster_buffer_provider.cc
+++ b/cc/raster/gpu_raster_buffer_provider.cc

@@ -17,6 +17,7 @@
 #include "base/strings/stringprintf.h"
 #include "base/trace_event/process_memory_dump.h"
 #include "base/trace_event/trace_event.h"
+#include "build/chromeos_buildflags.h"
 #include "cc/base/histograms.h"
 #include "cc/paint/display_item_list.h"
 #include "cc/paint/paint_canvas.h"
@@ -310,7 +311,7 @@
       texture_target_(backing->texture_target),
       texture_is_overlay_candidate_(backing->overlay_candidate),
       mailbox_(backing->mailbox) {
-#if defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_CHROMEOS_ASH)
   // Only do this in Chrome OS with OOP-R because:
   //   1) We will use this timestamp to measure raster scheduling delay and we
   //      only need to collect that data to assess the impact of hardware
@@ -554,7 +555,7 @@
       << "Why are we rastering a tile that's not dirty?";
 
   if (measure_raster_metric) {
-#if defined(OS_CHROMEOS)
+#if BUILDFLAG(IS_CHROMEOS_ASH)
     // Use a query to detect when the GPU side is ready to start issuing raster
     // work to the driver. We will use the resulting timestamp to measure raster
     // scheduling delay. We only care about this in Chrome OS and when OOP-R is