Revert of Re-land: base: Enable browser-wide discardable memory on Linux, CrOS...

Revert of Re-land: base: Enable browser-wide discardable memory on Linux, CrOS and Windows. (patchset #1 id:1 of https://codereview.chromium.org/793443003/)

Reason for revert:
Caused random crashes on GPU bots with this assertion failure:
WebglConformance.conformance_ogles_GL_log_log_001_to_008:
Error installing extension 'v8/gc'.
[3060:3996:0218/193241:FATAL:discardable_shared_memory_heap.cc(205)] Check failed: spans_.find(span->start_ + span->length_ - 1) != spans_.end().

Original issue's description:
> 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
>
> Committed: https://crrev.com/2f7a8e80b83aa581aa6de86117a692093d16c217
> Cr-Commit-Position: refs/heads/master@{#316977}

TBR=avi@chromium.org,danakj@chromium.org,reveman@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=429415,429416

Review URL: https://codereview.chromium.org/917023003

Cr-Commit-Position: refs/heads/master@{#316997}

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_SHMEM,
-    DISCARDABLE_MEMORY_TYPE_EMULATED
+    DISCARDABLE_MEMORY_TYPE_EMULATED,
+    DISCARDABLE_MEMORY_TYPE_SHMEM
   };
   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_SHMEM,
-    DISCARDABLE_MEMORY_TYPE_EMULATED
+    DISCARDABLE_MEMORY_TYPE_EMULATED,
+    DISCARDABLE_MEMORY_TYPE_SHMEM
   };
   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_SHMEM,
-    DISCARDABLE_MEMORY_TYPE_EMULATED
+    DISCARDABLE_MEMORY_TYPE_EMULATED,
+    DISCARDABLE_MEMORY_TYPE_SHMEM
   };
   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_SHMEM,
-    DISCARDABLE_MEMORY_TYPE_EMULATED
+    DISCARDABLE_MEMORY_TYPE_EMULATED,
+    DISCARDABLE_MEMORY_TYPE_SHMEM
   };
   types->assign(supported_types, supported_types + arraysize(supported_types));
 }