rebase_path() SK_USER_CONFIG_HEADER

So close... this is not one of the types of strings
that's auto rebase_path()'d, so do it ourself.

This time I've actually checked that building with

   #include SK_USER_CONFIG_HEADER

in SkTypes.h works.

Change-Id: Id9848f04412d34695094c35d8267dd177ec35ebd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1580040
Commit-Queue: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@chromium.org>
Reviewed-by: Brian Osman <brianosman@google.com>
Cr-Commit-Position: refs/heads/master@{#653475}

skia/BUILD.gn

@@ -56,7 +56,7 @@ config("skia_config") {
   defines = [
     "SK_HAS_PNG_LIBRARY",
     "SK_HAS_WEBP_LIBRARY",
-    "SK_USER_CONFIG_HEADER=\"config/SkUserConfig.h\"",
+    "SK_USER_CONFIG_HEADER=\"" + rebase_path("config/SkUserConfig.h") + "\"",
   ]
 
   if (!is_ios) {