added SkSL to Skia build

This fixes a pfdium build error when using the Skia backend, due to new Skia files which are not being properly included in the pdfium build.

Review-Url: https://codereview.chromium.org/2403863002
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index 43342b6..178b694 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -80,6 +80,7 @@
     "//third_party/skia/src/pdf",
     "//third_party/skia/src/ports",
     "//third_party/skia/src/sfnt",
+    "//third_party/skia/src/sksl",
     "//third_party/skia/src/utils",
     "//third_party/skia/src/lazy",
   ]
@@ -173,6 +174,7 @@
   # The skia gypi values are relative to the skia_dir, so we need to rebase.
   sources += skia_core_sources
   sources += skia_effects_sources
+  sources += skia_sksl_sources
   sources += skia_utils_sources
   sources += [
     "//third_party/skia/src/fonts/SkFontMgr_indirect.cpp",