Document use of TextZoomFactor().

Change-Id: I83c453ef425d239912775aac6db791cec6c13e09
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1800165
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Cr-Commit-Position: refs/heads/master@{#695976}

third_party/blink/renderer/core/css/resolver/font_builder.cc

@@ -238,7 +238,8 @@ float FontBuilder::GetComputedSizeFromSpecifiedSize(
     float specified_size) {
   DCHECK(document_);
   float zoom_factor = effective_zoom;
-  // FIXME: Why is this here!!!!?!
+  // Apply the text zoom factor preference. The preference is exposed in
+  // accessibility settings in Chrome for Android to improve readability.
   if (LocalFrame* frame = document_->GetFrame())
     zoom_factor *= frame->TextZoomFactor();