Use NextFireIntervalDelta instead of NextFireInterval in SVGImage*Test

Avoids run-tripping TimeDelta -> double -> TimeDelta.

Bug: 856082
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: If4a6a0626be6341770ef01d368fc8a1ddf00de31
Reviewed-on: https://chromium-review.googlesource.com/1126115
Reviewed-by: Daniel Bratell <bratell@opera.com>
Commit-Queue: Fredrik Söderquist <fs@opera.com>
Cr-Commit-Position: refs/heads/master@{#572560}
1 file changed