Do not invoke DrawFadedString() if it knows it does not fade.

Do not invoke DrawFadedString() if it knows it does not fade.

This is intentionally tiny and intended to be merged into M41.
views::Label already knows the size of the text (GetTextSize()
returns the cached size), therefore it knows the current text
does not fade at all.

Since this will avoid the check if the text may fade or not in
gfx::Canvas::DrawFadedString(), this can potentially make
the drawing 2x faster if the text doesn't fade at all.

BUG=451297
R=sky@chromium.org
TEST=on link_freon, with the bookmark sample in crbug.com/440551
  (contains ~15 bookmarks), repaint of bookmark bar is improved
  as: ~100msec -> ~55msec

Review URL: https://codereview.chromium.org/870903002

Cr-Commit-Position: refs/heads/master@{#312942}