Use CSS value instead of serialized string in CanvasRenderingContext2D::setFont

Use CSS value instead of serialized string in CanvasRenderingContext2D::setFont

This change improves the performance of setting a font to a value that is not
in the font string cache by optimizing the code that determines whether the
new value is 'inherit' or 'initial'. The current code is serializing
the font meta-property, and then doing a string comparison.
It is a lot cheaper to bypass the serialization and just check the class
of the font-size property.

BUG=497864
TEST=canvas/philip/tests/2d.text.font.parse.invalid.html + telemetry tough_canvas_cases/canvas-font-cycler.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197878

[Justin Novosad, 2015-06-25 20:52:15]

junov@chromium.org changed reviewers:
+ senorblanco@chromium.org

[Justin Novosad, 2015-06-25 20:52:16]

PTAL

[Stephen White, 2015-06-25 21:54:39]

LGTM

(I'm trusting you that checking font-size is semantically equivalent to checking
the whole font property -- I am not enough of a CSS guru to say.)

[Justin Novosad, 2015-06-26 02:45:47]

On 2015/06/25 21:54:39, Stephen White wrote:
> LGTM
> 
> (I'm trusting you that checking font-size is semantically equivalent to
checking
> the whole font property -- I am not enough of a CSS guru to say.)

I actually tracked that down by stepping through the serialization code to
figure out where the "inherited" string originates.  Also, we have layout test
coverage to validate this

[Justin Novosad, 2015-06-26 02:46:02]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2015-06-26 02:46:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1210273002/1

[commit-bot: I haz the power, 2015-06-26 02:50:11]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197878