Revert of win: Remove unneeded references to visual_studio_version now that it's always 2015.

build/config/win/BUILD.gn

Index: build/config/win/BUILD.gn
diff --git a/build/config/win/BUILD.gn b/build/config/win/BUILD.gn
index 3da27470120c5fe17c564f6f012dff4694cbc627..52ae78f9c5a9df981169da220ea41426f7934d8d 100644
--- a/build/config/win/BUILD.gn
+++ b/build/config/win/BUILD.gn
@@ -48,24 +48,36 @@ config("compiler") {
   cflags_c = [ "/TC" ]
   cflags_cc = [ "/TP" ]
 
-  cflags += [
-    # Tell the compiler to crash on failures. This is undocumented
-    # and unsupported but very handy.
-    "/d2FastFail",
-
-    # Work around crbug.com/526851, bug in VS 2015 RTM compiler.
-    "/Zc:sizedDealloc-",
-
-    # Disable thread-safe statics to avoid overhead and because
-    # they are disabled on other platforms. See crbug.com/587210
-    # and -fno-threadsafe-statics.
-    "/Zc:threadSafeInit-",
-  ]
+  # Flags not supported in version 2013.
+  if (visual_studio_version != "2013" && visual_studio_version != "2013e") {
+    cflags += [
+      # Tell the compiler to crash on failures. This is undocumented
+      # and unsupported but very handy.
+      "/d2FastFail",
+    ]
+  }
+
+  if (visual_studio_version == "2015") {
+    cflags += [
+      # Work around crbug.com/526851, bug in VS 2015 RTM compiler.
+      "/Zc:sizedDealloc-",
+
+      # Disable thread-safe statics to avoid overhead and because
+      # they are disabled on other platforms. See crbug.com/587210
+      # and -fno-threadsafe-statics.
+      "/Zc:threadSafeInit-",
+    ]
+  }
 
   # Building with Clang on Windows is a work in progress and very
   # experimental. See crbug.com/82385.
+  # Keep this in sync with the similar block in build/common.gypi
   if (is_clang) {
-    cflags += [ "-fmsc-version=1900" ]
+    if (visual_studio_version == "2013") {
+      cflags += [ "-fmsc-version=1800" ]
+    } else if (visual_studio_version == "2015") {
+      cflags += [ "-fmsc-version=1900" ]
+    }
 
     if (current_cpu == "x86") {
       cflags += [ "-m32" ]