Work around VC++ code-ben bug 804884

VC++ PGO builds crash deep inside memmove, as called by
message_center::InnerBoundedLabel::GetSizeForWidthAndLines which calls
base::JoinString and then ultimately basic_string<>::append and memmove.
Inspection of the stack and the generated code shows that
base::JoinString is called correctly but that base::JoinString (or the
inlined JoinStringT and three other levels of inlining) fail to read
separator off the stack correctly - they read it from an address that is
four bytes too early. This leads to copying too many bytes from the
wrong address.

Fully disabling optimizations in VC++ builds for the affected function
seems like the right fix. If time permits then a reduction and VC++ bug
will be created. However if this is a PGO-only bug then...

Bug: 804884
Change-Id: I80fd8e693a35b26a386cd99a1b17c54bede95d5c
Reviewed-on: https://chromium-review.googlesource.com/891569Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#532658}

base/strings/string_util.cc

@@ -936,6 +936,11 @@ char16* WriteInto(string16* str, size_t length_with_null) {
   return WriteIntoT(str, length_with_null);
 }
 
+#if defined(_MSC_VER) && !defined(__clang__)
+// Work around VC++ code-gen bug. https://crbug.com/804884
+#pragma optimize("", off)
+#endif
+
 // Generic version for all JoinString overloads. |list_type| must be a sequence
 // (std::vector or std::initializer_list) of strings/StringPieces (std::string,
 // string16, StringPiece or StringPiece16). |string_type| is either std::string
@@ -983,6 +988,11 @@ string16 JoinString(const std::vector<string16>& parts,
   return JoinStringT(parts, separator);
 }
 
+#if defined(_MSC_VER) && !defined(__clang__)
+// Work around VC++ code-gen bug. https://crbug.com/804884
+#pragma optimize("", on)
+#endif
+
 std::string JoinString(const std::vector<StringPiece>& parts,
                        StringPiece separator) {
   return JoinStringT(parts, separator);