GCC: do not use namespace linux in system memory pressure evaluator

GCC fails to compile system memory pressure evaluator, as the
linux implementation is in linux namespace. But in GCC, linux
is a reserved word, so it should not be used. To fix the
problem, we rename the new namespace to os_linux.

../../base/util/memory_pressure/system_memory_pressure_evaluator_linux.h:16:11: error: expected identifier before numeric constant
   16 | namespace linux {
      |           ^~~~~

Bug: 819294
Change-Id: I3edcbbbb91fe99aab8c0be952385bb38bb1e27e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2394158Reviewed-by: Sébastien Marchand <sebmarchand@chromium.org>
Commit-Queue: José Dapena Paz <jdapena@igalia.com>
Cr-Commit-Position: refs/heads/master@{#807099}

base/util/memory_pressure/system_memory_pressure_evaluator.cc

@@ -46,7 +46,7 @@ SystemMemoryPressureEvaluator::CreateDefaultSystemEvaluator(
   }
   return evaluator;
 #elif defined(OS_LINUX) && !defined(OS_CHROMEOS)
-  return std::make_unique<util::linux::SystemMemoryPressureEvaluator>(
+  return std::make_unique<util::os_linux::SystemMemoryPressureEvaluator>(
       monitor->CreateVoter());
 #endif
   return nullptr;

base/util/memory_pressure/system_memory_pressure_evaluator_linux.cc

@@ -32,7 +32,7 @@ int GetAvailableSystemMemoryMiB(const base::SystemMemoryInfoKB& mem_info) {
 }  // namespace
 
 namespace util {
-namespace linux {
+namespace os_linux {
 
 const base::TimeDelta SystemMemoryPressureEvaluator::kModeratePressureCooldown =
     base::TimeDelta::FromSeconds(10);
@@ -153,5 +153,5 @@ SystemMemoryPressureEvaluator::CalculateCurrentPressureLevel() {
   return base::MemoryPressureListener::MEMORY_PRESSURE_LEVEL_NONE;
 }
 
-}  // namespace linux
+}  // namespace os_linux
 }  // namespace util

base/util/memory_pressure/system_memory_pressure_evaluator_linux.h

@@ -13,7 +13,7 @@
 #include "base/util/memory_pressure/system_memory_pressure_evaluator.h"
 
 namespace util {
-namespace linux {
+namespace os_linux {
 
 // Linux memory pressure voter. Because there is no OS provided signal this
 // polls at a low frequency, and applies internal hysteresis.
@@ -106,7 +106,7 @@ class SystemMemoryPressureEvaluator
   SEQUENCE_CHECKER(sequence_checker_);
 };
 
-}  // namespace linux
+}  // namespace os_linux
 }  // namespace util
 
 #endif  // BASE_UTIL_MEMORY_PRESSURE_SYSTEM_MEMORY_PRESSURE_EVALUATOR_LINUX_H_

base/util/memory_pressure/system_memory_pressure_evaluator_linux_unittest.cc

@@ -13,7 +13,7 @@
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace util {
-namespace linux {
+namespace os_linux {
 
 namespace {
 
@@ -257,5 +257,5 @@ TEST_F(LinuxSystemMemoryPressureEvaluatorTest, CheckMemoryPressure) {
   testing::Mock::VerifyAndClearExpectations(&evaluator);
 }
 
-}  // namespace linux
+}  // namespace os_linux
 }  // namespace util