Change tab offset constant to tab overlap constant and add getter in TabStrip.
Referring to the overlap between two tabs makes more sense intuitively than
talking about the offset of the second tab's left edge from the first tab's end.
It's also a little clearer in code since you're never subtracting a negative
number.
The getter in TabStrip isn't currently interesting (just returns a constant),
but will make more sense once this value is parametrized for the Material
Design work.
BUG=none
TEST=none
Review URL: https://codereview.chromium.org/
1327233002
Cr-Commit-Position: refs/heads/master@{#348189}