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Before this CL, RenderText::GetSubstringBounds() rounded up both the
starting x and ending x that it computed. As a result, the left edge
of the substring could be slightly outside the returned bounds, and
the right edge of the returned bounds might contain some small portion
of the next characters.

When the output of GetSubstringBounds() is used as input for
set_display_rect(), this rounding behavior could show up as small
artifacts trailing the domain and/or clipping the first character
slightly (see screenshots in linked bug).

This CL makes GetSubstringBounds() round down the starting x
coordinate.  This provides the guarantee that the returned bounds
always fully contain the substring -- that is, the bounds will be
slightly larger rather than slightly smaller when rounding occurs. An
alternative I considered was to introduce GetSubstringBoundsF and let
consumers do their own rounding. However, as far as I can tell no
other caller cares about the rounding behavior so it seemed like
unnecessary complexity to add a float version of the method.

Bug: 1108045, 1111044
Change-Id: I733326ef0544c4ecbe3e39bb9bd8f7a410bc0047
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2317820
Commit-Queue: Emily Stark <estark@chromium.org>
Reviewed-by: Michael Wasserman <msw@chromium.org>
Reviewed-by: Etienne Bergeron <etienneb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#793692}