DevTools: Use geometry instead of CSS for code folding hover

The previous CL's CSS hack incorrectly cut off breakpoints, caused the
gutter to be transparent when scrolling code horizontally, and made
closed folding triangles invisible.

Bug: 328431
Change-Id: Iba9482efd4bfd13d750580746f5507dadd0c73b2
Reviewed-on: https://chromium-review.googlesource.com/c/1380557
Reviewed-by: Erik Luo <luoe@chromium.org>
Commit-Queue: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#617362}
3 files changed