Fix crash when opening GTK file dialogs in component builds

This fixes a regression after f4d4d111 which rolled in the addition of
metrics for harfbuzz.  The issue is that the system harfbuzz and our
//third_party harfbuzz were getting loaded at the same time.  This CL
adds HB metrics for component builds to prevent loading the system
harfbuzz.  This change is not done for non-component builds because
it's not necessary and we don't want to accidentally add a dependency
on HB's metrics.

R=drott

Bug: 1146397
Change-Id: Ifaebf0c86bedefb29cc88709eb89fb42af5a7b75
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2522922
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/master@{#826995}

third_party/harfbuzz-ng/BUILD.gn

@@ -280,6 +280,15 @@ if (use_system_harfbuzz) {
       } else {
         configs -= [ "//build/config/gcc:symbol_visibility_hidden" ]
         configs += [ "//build/config/gcc:symbol_visibility_default" ]
+
+        # Exposing symbols in component builds can cause a mixture of system
+        # harfbuzz symbols and //third_party harfbuzz symbols to be loaded at
+        # the same time.  This can cause issues for libraries that indirectly
+        # load harfbuzz like pango (see https://crbug.com/1146397), so include
+        # metrics symbols to prevent pango from using the system library.
+        # TODO(https://crbug.com/1148459): Remove this workaround once bug
+        # 1148459 is fixed.
+        defines -= [ "HB_NO_METRICS" ]
       }
     }