Use placement new to allocate GPU command buffer entries.

This isn't as ugly as the crazy null check, and it seems to be compiled correctly.

BUG=

Review URL: https://codereview.chromium.org/427003008

Cr-Commit-Position: refs/heads/master@{#288481}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@288481 0039d316-1c4b-4281-b951-d872f2087c98

gpu/command_buffer/client/cmd_buffer_helper.h

@@ -135,27 +135,13 @@ class GPU_EXPORT CommandBufferHelper {
     return space;
   }
 
-  template <typename T>
-  void ForceNullCheck(T* data) {
-#if defined(OS_WIN) && defined(ARCH_CPU_64_BITS)
-    // 64-bit MSVC's alias analysis was determining that the command buffer
-    // entry couldn't be NULL, so it optimized out the NULL check.
-    // Dereferencing the same datatype through a volatile pointer seems to
-    // prevent that from happening. http://crbug.com/361936
-    if (data)
-      static_cast<volatile T*>(data)->header;
-#endif
-  }
-
   // Typed version of GetSpace. Gets enough room for the given type and returns
   // a reference to it.
   template <typename T>
   T* GetCmdSpace() {
     COMPILE_ASSERT(T::kArgFlags == cmd::kFixed, Cmd_kArgFlags_not_kFixed);
     int32 space_needed = ComputeNumEntries(sizeof(T));
-    T* data = static_cast<T*>(GetSpace(space_needed));
-    ForceNullCheck(data);
-    return data;
+    return new (GetSpace(space_needed)) T;
   }
 
   // Typed version of GetSpace for immediate commands.
@@ -163,9 +149,7 @@ class GPU_EXPORT CommandBufferHelper {
   T* GetImmediateCmdSpace(size_t data_space) {
     COMPILE_ASSERT(T::kArgFlags == cmd::kAtLeastN, Cmd_kArgFlags_not_kAtLeastN);
     int32 space_needed = ComputeNumEntries(sizeof(T) + data_space);
-    T* data = static_cast<T*>(GetSpace(space_needed));
-    ForceNullCheck(data);
-    return data;
+    return new (GetSpace(space_needed)) T;
   }
 
   // Typed version of GetSpace for immediate commands.
@@ -173,9 +157,7 @@ class GPU_EXPORT CommandBufferHelper {
   T* GetImmediateCmdSpaceTotalSize(size_t total_space) {
     COMPILE_ASSERT(T::kArgFlags == cmd::kAtLeastN, Cmd_kArgFlags_not_kAtLeastN);
     int32 space_needed = ComputeNumEntries(total_space);
-    T* data = static_cast<T*>(GetSpace(space_needed));
-    ForceNullCheck(data);
-    return data;
+    return new (GetSpace(space_needed)) T;
   }
 
   int32 last_token_read() const {