Replace O(n^2) set intersection test with O(n)

Replace O(n^2) set intersection test with O(n)

The original Blink code used unordered WTF sets, and we missed this
when porting the back end to Chromium and std::set.

BUG=280741
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=220416

[jsbell, 2013-08-28 22:57:53]

Odds are we'll never notice the performance difference, but it doesn't introduce
code complexity and I'll sleep better.

Overlapping scopes are well exercised by Blink layout tests, so I didn't expose
this function outside the file and implement a unit test, but I can if desired.

[dgrogan, 2013-08-28 23:05:29]

LGTM

Did you overlook or reject std::set_intersection?

[jsbell, 2013-08-28 23:36:37]

On 2013/08/28 23:05:29, dgrogan wrote:
> LGTM
> 
> Did you overlook or reject std::set_intersection?

It copies the elements into an output structure, which is unnecessary here.

It wasn't apparent how to use that for a simple Boolean test that could exit
early.

[commit-bot: I haz the power, 2013-08-29 18:24:34]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/jsbell@chromium.org/23490012/1

[commit-bot: I haz the power, 2013-08-29 23:03:29]

Change committed as 220416