Some iwyu for font_custom_platform_data.h.

FontCustomPlatformData::FamilyNameForInspector returns an SkString so
this adds an include for SkString.h. Previously this code compiled
because SkTypes.h has a forward declaration of SkString which was pulled
in though the SkRefCnt.h include. Skia is looking to remove this forward
declaration in SkTypes.h.

Change-Id: I638e13a0db4fba02e9caf73b448aa70ae2011100
Reviewed-on: https://chromium-review.googlesource.com/1099416Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Ben Wagner <bungeman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#566930}

third_party/blink/renderer/platform/fonts/font_custom_platform_data.h

@@ -41,6 +41,7 @@
 #include "third_party/blink/renderer/platform/wtf/ref_counted.h"
 #include "third_party/blink/renderer/platform/wtf/text/wtf_string.h"
 #include "third_party/skia/include/core/SkRefCnt.h"
+#include "third_party/skia/include/core/SkString.h"
 
 class SkTypeface;