Remove obsolete webkit_box_lines.h reference

This is causing ninja to treat WebKit as dirty on every compile, for
certain build configurations. Follow-up to https://crrev.com/c/957264.

Bug: 818691
Change-Id: Idd32f722b8d4efe82388a06e2acc0d638cd9eb4e
Reviewed-on: https://chromium-review.googlesource.com/961784Reviewed-by: Ian Kilpatrick <ikilpatrick@chromium.org>
Commit-Queue: Michael Giuffrida <michaelpg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542988}

third_party/WebKit/Source/core/BUILD.gn

@@ -888,7 +888,6 @@ css_properties("make_core_generated_css_longhand_property_classes") {
     "$blink_core_output_dir/css/properties/longhands/webkit_box_direction.h",
     "$blink_core_output_dir/css/properties/longhands/webkit_box_flex.h",
     "$blink_core_output_dir/css/properties/longhands/webkit_box_flex_group.h",
-    "$blink_core_output_dir/css/properties/longhands/webkit_box_lines.h",
     "$blink_core_output_dir/css/properties/longhands/webkit_box_ordinal_group.h",
     "$blink_core_output_dir/css/properties/longhands/webkit_box_orient.h",
     "$blink_core_output_dir/css/properties/longhands/webkit_box_pack.h",