Take default system font size from PlatformFont

The default font returned by Skia should take the initial size from the
default value kDefaultBaseFontSize specified in PlatformFont.

R=robliao@chromium.org, asvitkine@chromium.org
CC=benck@google.com

Bug: 944227
Change-Id: I6b230b80c349abbe5968edb3cebdd6e89db4c4a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1642738
Reviewed-by: Robert Liao <robliao@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666299}
PlatformFont* PlatformFont::CreateDefault() {
  return new PlatformFontSkia;
}

PlatformFont* PlatformFont::CreateDefault() {
  return new PlatformFontSkia;
}
