Disable MSVC optimization of sk_tool_utils::make_big_path().

This one function takes MSVC about 2 minutes to compile with optimization turned on, and roughly doubles my clean build wall time on a Z840 (186s before this change -> 95s after).

It's test-only, so who really cares how fast it is?

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3254

Change-Id: I1318024caee281f3f8d44c05258b2bd4bd4eb695
Reviewed-on: https://skia-review.googlesource.com/3254
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>

tools/sk_tool_utils.cpp

@@ -365,6 +365,11 @@ void create_tetra_normal_map(SkBitmap* bm, const SkIRect& dst) {
     }
 }
 
+#if defined(_MSC_VER)
+    // MSVC takes ~2 minutes to compile this function with optimization.
+    // We don't really care to wait that long for this function.
+    #pragma optimize("", off)
+#endif
 void make_big_path(SkPath& path) {
     #include "BigPathBench.inc"
 }