Cleanup platform_font header file

This CL is removing useless forward declaration and unused include file.

There is no behavior change.

R=robliao@chromium.org, asvitkine@chromium.org
CC=​benck@google.com

Bug: 944227
Change-Id: Ifcda56f000119dd3682eff2e7665f59a49c84ed1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1643381Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666282}

ui/gfx/platform_font.h

@@ -8,7 +8,6 @@
 #include <string>
 
 #include "base/memory/ref_counted.h"
-#include "base/strings/string16.h"
 #include "build/build_config.h"
 #include "ui/gfx/font.h"
 #include "ui/gfx/gfx_export.h"
@@ -16,9 +15,6 @@
 
 namespace gfx {
 
-class Font;
-struct FontRenderParams;
-
 class GFX_EXPORT PlatformFont : public base::RefCounted<PlatformFont> {
  public:
 // The size of the font returned by CreateDefault() on a "default" platform