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-[TabView drawRect:] was called repeatedly to unnecessarily update the glow
that is not changing or should not have changed.

1. Selected tabs don't have glow, so don't redraw on mouseMoved.

2. When dragging an unselected tab, the mouse point relative to the tab view
does not change, so no need to redraw the glow.

3. When a dragged unselected tab slides behind another tab, let the dragged tab
keep the glow instead of the tab in front of it (this is a behavior change).
I think this is the more correct behavior, and we never have to redraw tabs
while dragging.

BUG=452925

Review URL: https://codereview.chromium.org/882193003

Cr-Commit-Position: refs/heads/master@{#313734}