Don't create MemoryPressureMonitor for Chromecast

Chromecast creates MemoryPressureMonitor and evaluator itself in
CastBrowserMainParts::PreMainMessageLoopRun and that it conflicts with
Chromium adding a MemoryPressureMonitor in the following CL and causes a
fatal error when creating MemoryPressureMonitor:
https://chromium-review.googlesource.com/c/chromium/src/+/1386856

Bug: 1125990
Test: Pass CQ
Change-Id: I8fb76f6fcbc725b9b5e252898dc8d75a17d41bf0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397990Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Chih-Hsuan Kuo <chkuo@google.com>
Auto-Submit: Chih-Hsuan Kuo <chkuo@google.com>
Cr-Commit-Position: refs/heads/master@{#805215}

content/browser/BUILD.gn

@@ -51,6 +51,7 @@ source_set("browser") {
     "//base/util/ranges:ranges",
     "//build:branding_buildflags",
     "//build:chromeos_buildflags",
+    "//build:chromecast_buildflags",
     "//cc",
     "//cc/animation",
     "//cc/mojo_embedder",

content/browser/browser_main_loop.cc

@@ -50,6 +50,7 @@
 #include "base/util/memory_pressure/multi_source_memory_pressure_monitor.h"
 #include "build/branding_buildflags.h"
 #include "build/build_config.h"
+#include "build/chromecast_buildflags.h"
 #include "cc/base/histograms.h"
 #include "components/discardable_memory/service/discardable_shared_memory_manager.h"
 #include "components/services/storage/dom_storage/storage_area_impl.h"
@@ -366,7 +367,7 @@ std::unique_ptr<base::MemoryPressureMonitor> CreateMemoryPressureMonitor(
   if (chromeos::switches::MemoryPressureHandlingEnabled())
     monitor = std::make_unique<util::MultiSourceMemoryPressureMonitor>();
 #elif defined(OS_MAC) || defined(OS_WIN) || defined(OS_FUCHSIA) || \
-    (defined(OS_LINUX) && !defined(OS_CHROMEOS))
+    (defined(OS_LINUX) && !defined(OS_CHROMEOS) && !BUILDFLAG(IS_CHROMECAST))
   monitor = std::make_unique<util::MultiSourceMemoryPressureMonitor>();
 #endif
   // No memory monitor on other platforms...