Fix a clobber build issue on Linux.

"R.h" in core/BUILD.gn should be "r.h".
This is a follow-up of crrev.com/541035.

TBR=nainar@chromium.org
NOTRY=true

Bug: 770603
Change-Id: I89efbac7dbaccf263344766b64610b1b822a85d8
Reviewed-on: https://chromium-review.googlesource.com/950524Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541038}

third_party/WebKit/Source/core/BUILD.gn

@@ -764,7 +764,7 @@ css_properties("make_core_generated_css_longhand_property_classes") {
     "$blink_core_output_dir/css/properties/longhands/pointer_events.h",
     "$blink_core_output_dir/css/properties/longhands/position.h",
     "$blink_core_output_dir/css/properties/longhands/quotes.h",
-    "$blink_core_output_dir/css/properties/longhands/R.h",
+    "$blink_core_output_dir/css/properties/longhands/r.h",
     "$blink_core_output_dir/css/properties/longhands/resize.h",
     "$blink_core_output_dir/css/properties/longhands/right.h",
     "$blink_core_output_dir/css/properties/longhands/rotate.h",