Handle non-image backgrounds in BoxModelObjectPainter::PaintTextClipMask

When we paint a background color for a line, we don't adjust the paint
offset based on the logical offset on the "virtual line", so we mustn't
try to undo the adjustment in BoxModelObjectPainter::PaintTextClipMask
when we're generating the mask for "(-webkit-)background-clip: text".

Check the |object_has_multiple_boxes| and don't undo the adjustment if
it is false.

Bug: 879292
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9ec9c5cdfeb675803e54407f2e98e39b2dc1bc6b
Reviewed-on: https://chromium-review.googlesource.com/1199343
Reviewed-by: Stephen Chenney <schenney@chromium.org>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#588038}
3 files changed