Remove Mac OS 10.10 specific forward declaration

After removing support for Mac OS 10.10, remove specific workaround to
access a non-public font function.

Fixed: 1150869
Cq-Include-Trybots: luci.chromium.try:mac11.0-blink-rel,mac10.15-blink-rel,mac10.14-blink-rel,mac10.13-blink-rel,mac10.12-blink-rel
Change-Id: I41321d03bd97a42c53d8d6012b79ebdbe6ca1d00
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2549953Reviewed-by: Koji Ishii <kojii@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829308}

third_party/blink/renderer/platform/fonts/mac/font_matcher_mac.mm

@@ -42,10 +42,6 @@
 #import "third_party/blink/renderer/platform/wtf/hash_set.h"
 #import "third_party/blink/renderer/platform/wtf/text/atomic_string_hash.h"
 
-@interface NSFont (YosemiteAdditions)
-+ (NSFont*)systemFontOfSize:(CGFloat)size weight:(CGFloat)weight;
-@end
-
 namespace {
 
 static CGFloat toFontWeight(blink::FontSelectionValue font_weight) {
@@ -176,15 +172,9 @@ NSFont* MatchNSFontFamily(const AtomicString& desired_family_string,
 
   if (desired_family_string == font_family_names::kSystemUi) {
     NSFont* font = nil;
-// Normally we'd use an availability macro here, but
-// systemFontOfSize:weight: is available but not visible on macOS 10.10,
-// so it's been forward declared earlier in this file.
-// On OSX 10.10+, the default system font has more weights.
-#pragma clang diagnostic push
-#pragma clang diagnostic ignored "-Wunguarded-availability"
-    font = [NSFont systemFontOfSize:size weight:toFontWeight(desired_weight)];
-#pragma clang diagnostic pop
-
+    if (@available(macOS 10.11, *)) {
+      font = [NSFont systemFontOfSize:size weight:toFontWeight(desired_weight)];
+    }
     if (desired_traits & IMPORTANT_FONT_TRAITS)
       font = [[NSFontManager sharedFontManager] convertFont:font
                                                 toHaveTrait:desired_traits];