Don't adopt a null FontSelector in CanvasRenderingContext2D::setFont()

This function does the following:
1. Create a temporary ComputedStyle |font_style|
2. Make |font_style| adopt its current FontSelector, which is always
   nullptr at the call site
3. Use StyleResolver::ComputeFont() to resolve font-related properties,
   which overrides the FontSelector of |font_style| with the underlying
   document's FontSelector

This isn't correct, as it switches the FontSelector of |font_style|,
while ComputeFont() can already use the document's FontSelector. The
null FontSelector is actually never used.

Hence, this patch removes the adoption of null FontSelector.

Bug: 1049295
Change-Id: I2d3b2d1affd9317c6535933c1e4cc2421f5be1c6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103258Reviewed-by: Fernando Serboncini <fserb@chromium.org>
Commit-Queue: Xiaocheng Hu <xiaochengh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750625}

third_party/blink/renderer/modules/canvas/canvas2d/canvas_rendering_context_2d.cc

@@ -539,7 +539,6 @@ void CanvasRenderingContext2D::setFont(const String& new_font) {
           element_font_description.SpecifiedSize());
 
       font_style->SetFontDescription(element_font_description);
-      font_style->GetFont().Update(font_style->GetFont().GetFontSelector());
       canvas()->GetDocument().EnsureStyleResolver().ComputeFont(
           *canvas(), font_style.get(), *parsed_style);