Rename style_builder_custom.cc -> style_builder.cc.

"Custom" is no longer appropriate for this file, now that we don't
generate style_builder.cc.

Note: style_builder.h was missing from BUILD.gn for some reason.

R=futhark@chromium.org

Bug: 751354
Change-Id: Ie2ac69734979a9b9488e876533da51b68e6ae971
Reviewed-on: https://chromium-review.googlesource.com/1079387Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Anders Ruud <andruud@chromium.org>
Cr-Commit-Position: refs/heads/master@{#562970}

third_party/blink/renderer/core/css/BUILD.gn

@@ -856,9 +856,10 @@ blink_core_sources("css") {
     "resolver/selector_filter_parent_scope.h",
     "resolver/style_adjuster.cc",
     "resolver/style_adjuster.h",
+    "resolver/style_builder.cc",
+    "resolver/style_builder.h",
     "resolver/style_builder_converter.cc",
     "resolver/style_builder_converter.h",
-    "resolver/style_builder_custom.cc",
     "resolver/style_resolver.cc",
     "resolver/style_resolver.h",
     "resolver/style_resolver_state.cc",