[Paint Preview] Conditionally enable CFF subsetting

Enable/compile CFF subsetting in harfbuzz when the
enable_paint_preview arg is set. This ensures the binary size increase
from enabling CFF font subsetting only occurs when Paint Previews (the
only feature using it) is compiled into Chrome.

Bug: 1031164
Change-Id: Ibfe8c0f0cca14bb1a28ce60f8fa41bc05ec062a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1952395Reviewed-by: Dominik Röttsches <drott@chromium.org>
Commit-Queue: Calder Kitagawa <ckitagawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#725574}

third_party/harfbuzz-ng/BUILD.gn

@@ -6,6 +6,7 @@ import("//build/config/chrome_build.gni")
 import("//build/config/features.gni")
 import("//build/config/freetype/freetype.gni")
 import("//build/config/ui.gni")
+import("//components/paint_preview/buildflags/buildflags.gni")
 import("//testing/libfuzzer/fuzzer_test.gni")
 import("//third_party/harfbuzz-ng/harfbuzz.gni")
 
@@ -261,6 +262,13 @@ if (use_system_harfbuzz) {
       "U_DISABLE_VERSION_SUFFIX=0",
     ]
 
+    if (enable_paint_preview) {
+      # Paint Previews make use of CFF subsetting. However, enabling this is
+      # expensive for binary size so only compile it when Paint Previews are
+      # compiled.
+      defines -= [ "HB_NO_SUBSET_CFF" ]
+    }
+
     if (is_component_build) {
       if (is_win) {
         defines += [ "HB_EXTERN=__declspec (dllexport) extern" ]