GrTessellator (AA): restore rounding in Line::intersect().
GrTessellator (AA): restore rounding in Line::intersect().

I removed this in https://skia-review.googlesource.com/8028, but that
was actually just masking a problem properly fixed in
https://skia-review.googlesource.com/8364. Now that it is fixed, we
can restore the rounding, which actually improves performance by
5-10% on Canvas Arcs (when run with the tess verb limit disabled).

NOTE: this change will affect many GMs, including strokes_poly,
addarc, parsedpaths, dashcubics, beziers, nested_aa, etc.

BUG=skia:

Change-Id: Id10f82e35a344247cab27e6d59a0dd2e11c9944d
Reviewed-on: https://skia-review.googlesource.com/9051
Reviewed-by: Brian Salomon <61fabaaf0876044aeedfd8e351e485c028d69e7b@google.com>
Commit-Queue: Stephen White <30b0ee0ef1651e8ba59cfd41f16e23abfe9aca8a@chromium.org>

google/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,aosp-mirror/platform_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,HalCanary/skia-hc,HalCanary/skia-hc,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,HalCanary/skia-hc,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia