include header as GN dependency

Bug: 661774
Change-Id: I7af30f80306fc506ab3b0b0b3eeed5b79fbbd845
Reviewed-on: https://chromium-review.googlesource.com/574632
Commit-Queue: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487144}

third_party/WebKit/Source/platform/BUILD.gn

@@ -740,6 +740,7 @@ component("platform") {
     "fonts/shaping/CaseMappingHarfBuzzBufferFiller.h",
     "fonts/shaping/HarfBuzzFace.cpp",
     "fonts/shaping/HarfBuzzFace.h",
+    "fonts/shaping/HarfBuzzFontCache.h",
     "fonts/shaping/HarfBuzzShaper.cpp",
     "fonts/shaping/HarfBuzzShaper.h",
     "fonts/shaping/RunSegmenter.cpp",