Simplify a call site of Font::Update() in CanvasFontCache

The site creates a new ComputedStyle(), and then immediately uses its
FontSelector, which is always nullptr at the moment, to call
Font::Update(). Hence, this patch simpilies it with nullptr.

This is preparation for eliminating Font::Update().

Bug: 1049295
Change-Id: I7279993d62c0a6a25e34fbfce8b5cd3dc6e6a3f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103647Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750627}

third_party/blink/renderer/core/html/canvas/canvas_font_cache.cc

@@ -37,8 +37,7 @@ CanvasFontCache::CanvasFontCache(Document& document)
   default_font_description.SetComputedSize(defaultFontSize);
   default_font_style_ = ComputedStyle::Create();
   default_font_style_->SetFontDescription(default_font_description);
-  default_font_style_->GetFont().Update(
-      default_font_style_->GetFont().GetFontSelector());
+  default_font_style_->GetFont().Update(nullptr);
 }
 
 CanvasFontCache::~CanvasFontCache() {