DevTools: Simplify mergeOrIntersect

Drive by:
1. The original mergeOrIntersect had a bug causing e.g. []x[1,2,3] produce [1,2,3].
2. findFilesMatchingSearchRequest had a bug that relied on #1 and produced almost correct results.

Both are fixed with this patch.

R=pfeldman@chromium.org, vsevik@chromium.org

Review URL: https://codereview.chromium.org/166703003

git-svn-id: svn://svn.chromium.org/blink/trunk@167282 bbb929c8-8fbe-4397-9dbb-9b2b20218538