[Fontations] Build Fontations font manager by default

SkFontMgr_Fontations is needed in fallback font instantiation in Blink
when moving system fonts to Fontations. When fallback font information
is retrieved via Mojo, the fallback font will be instantiated through
the default font mgr (FreeType, current) or the Fontations manager.

Bug: chromium:346918516
Change-Id: I805df1b80dd10d9c7e236a9cda461b342cbfa11a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6198511
Commit-Queue: Kaylee Lubick <kjlubick@chromium.org>
Reviewed-by: Kaylee Lubick <kjlubick@chromium.org>
Auto-Submit: Dominik Röttsches <drott@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1411681}

skia/BUILD.gn

@@ -418,6 +418,7 @@ component("skia") {
     }
     sources += skia_ports_typeface_proxy_sources
     sources += skia_ports_typeface_fontations_sources
+    sources += skia_ports_fontmgr_fontations_sources
 
     # Fontations Rust/C++ bridge interfaces.
     deps += [