Fix memory error in CanvasRenderingContext2D::drawTextInternal.

drawingContext() returns a raw ptr to an object that's OwnPtr'ed by HTMLCanvasElement.
CanvasRenderingContext2D::drawTextInternal saves that on the stack, then does
accessFont() which does updateStyleIgnorePendingStylesheets() which runs script from
autofocus that then blows away the GraphicsContext by doing setAttribute on the
canvas (.width = ...).

Instead, updateStyleIfNeeded before grabbing the GraphicsContext.

BUG=312050

Review URL: https://codereview.chromium.org/47933007

git-svn-id: svn://svn.chromium.org/blink/trunk@160780 bbb929c8-8fbe-4397-9dbb-9b2b20218538