Re-land: base: Enable browser-wide discardable memory on Linux, CrOS and Windows.

This makes SHMEM implementation of discardable memory preferred
over EMULATED implementation. This effectively makes SHMEM the
implementation used by default on Linux, CrOS and Windows.

SHMEM implementation of discardable memory gives the browser
process control over the total amount of discardable memory used
and allows us to enforce a global limit of 512MB across all
renderers.

BUG=429415,429416

Review URL: https://codereview.chromium.org/793443003

Cr-Commit-Position: refs/heads/master@{#316977}

base/memory/discardable_memory_android.cc

@@ -53,8 +53,8 @@ void DiscardableMemory::GetSupportedTypes(
     std::vector<DiscardableMemoryType>* types) {
   const DiscardableMemoryType supported_types[] = {
     DISCARDABLE_MEMORY_TYPE_ASHMEM,
-    DISCARDABLE_MEMORY_TYPE_EMULATED,
-    DISCARDABLE_MEMORY_TYPE_SHMEM
+    DISCARDABLE_MEMORY_TYPE_SHMEM,
+    DISCARDABLE_MEMORY_TYPE_EMULATED
   };
   types->assign(supported_types, supported_types + arraysize(supported_types));
 }

base/memory/discardable_memory_linux.cc

@@ -19,8 +19,8 @@ bool DiscardableMemory::ReduceMemoryUsage() {
 void DiscardableMemory::GetSupportedTypes(
     std::vector<DiscardableMemoryType>* types) {
   const DiscardableMemoryType supported_types[] = {
-    DISCARDABLE_MEMORY_TYPE_EMULATED,
-    DISCARDABLE_MEMORY_TYPE_SHMEM
+    DISCARDABLE_MEMORY_TYPE_SHMEM,
+    DISCARDABLE_MEMORY_TYPE_EMULATED
   };
   types->assign(supported_types, supported_types + arraysize(supported_types));
 }

base/memory/discardable_memory_mac.cc

@@ -23,8 +23,8 @@ void DiscardableMemory::GetSupportedTypes(
     std::vector<DiscardableMemoryType>* types) {
   const DiscardableMemoryType supported_types[] = {
     DISCARDABLE_MEMORY_TYPE_MACH,
-    DISCARDABLE_MEMORY_TYPE_EMULATED,
-    DISCARDABLE_MEMORY_TYPE_SHMEM
+    DISCARDABLE_MEMORY_TYPE_SHMEM,
+    DISCARDABLE_MEMORY_TYPE_EMULATED
   };
   types->assign(supported_types, supported_types + arraysize(supported_types));
 }

base/memory/discardable_memory_win.cc

@@ -19,8 +19,8 @@ bool DiscardableMemory::ReduceMemoryUsage() {
 void DiscardableMemory::GetSupportedTypes(
     std::vector<DiscardableMemoryType>* types) {
   const DiscardableMemoryType supported_types[] = {
-    DISCARDABLE_MEMORY_TYPE_EMULATED,
-    DISCARDABLE_MEMORY_TYPE_SHMEM
+    DISCARDABLE_MEMORY_TYPE_SHMEM,
+    DISCARDABLE_MEMORY_TYPE_EMULATED
   };
   types->assign(supported_types, supported_types + arraysize(supported_types));
 }