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This change sees single-line text rendering properly update the vertical
dimension of the underlying texture.

In addition, the recently-added text unittest is reworked to allow more
thorough and easy testing, by separating text layout from rendering and
canvas, and also exposing the internal text layout to tests.

BUG=
R=bshe

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Ib0ec5b678adf2727d58543e281685ce49ae691af
Reviewed-on: https://chromium-review.googlesource.com/801955Reviewed-by: Biao She <bshe@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521095}