style: Fix nsStyleBorder::mBorderColor for GCC.

GCC doesn't like StyleComplexColor with constructor in an anonymous
struct in an anonymous union.  Replace the use of a union to access
`mBorder[..]Color` fields as an array with an accessor methods.

Bug: 1465307
Reviewed-by: xidorn
MozReview-Commit-ID: 1Wulh1qKYCZ

components/style/properties/gecko.mako.rs

@@ -388,17 +388,9 @@ impl ${style_struct.gecko_struct_name} {
 
 <%!
 def get_gecko_property(ffi_name, self_param = "self"):
-    if "mBorderColor" in ffi_name:
-        return ffi_name.replace("mBorderColor",
-                                "unsafe { *%s.gecko.__bindgen_anon_1.mBorderColor.as_ref() }"
-                                % self_param)
     return "%s.gecko.%s" % (self_param, ffi_name)
 
 def set_gecko_property(ffi_name, expr):
-    if "mBorderColor" in ffi_name:
-        ffi_name = ffi_name.replace("mBorderColor",
-                                    "*self.gecko.__bindgen_anon_1.mBorderColor.as_mut()")
-        return "unsafe { %s = %s };" % (ffi_name, expr)
     return "self.gecko.%s = %s;" % (ffi_name, expr)
 %>
 
@@ -1594,7 +1586,7 @@ fn static_assert() {
         self.gecko.mComputedBorder.${side.ident} = self.gecko.mBorder.${side.ident};
     }
 
-    <% impl_color("border_%s_color" % side.ident, "(mBorderColor)[%s]" % side.index) %>
+    <% impl_color("border_%s_color" % side.ident, "mBorder%sColor" % side.name) %>
 
     <% impl_non_negative_length("border_%s_width" % side.ident,
                                 "mComputedBorder.%s" % side.ident,