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Due to a misunderstanding on my part, the initial implementation represented the "waiting" state, which is used while waiting for a network response in a Chrome tab. Most throbbers should use the "normal"/"spinning" state. This ports the CSS animations in MD_spinner_blue.svg to skia commands, and uses it in the card unmasking dialog.

After some investigation, I've realized that very few places in Chrome actually use the Throbber class. Tabs, for example, draw directly onto a canvas. So the next step will be to refactor this functionality into a ThrobberPainter class so tabs can use it. The reason I am not already doing this is I want to verify with the designers that this version looks good before I work on bringing it to more of Chrome.

BUG=259556, 473342

Review URL: https://codereview.chromium.org/1098853005

Cr-Commit-Position: refs/heads/master@{#327105}