ShapeDetection: add support for Text Detection in Mac

This CL adds text detection for Mac in services/shape_detection,
and also adds unittest for it, that will be run, like the
other specific unittests, in the mac gpu bots.

BUG=697925
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.mac:mac_optional_gpu_tests_rel
TEST= unittests that can be run anywhere like this:
out/gn/service_unittests  --gtest_filter=*TextDetection* --use-gpu-in-tests

and demo page, e.g.
./out/gn/Chromium.app/Contents/MacOS/Chromium  --enable-blink-features=ShapeDetection https://codepen.io/miguelao/full/ygxVqg

Review-Url: https://codereview.chromium.org/2726843006
Cr-Commit-Position: refs/heads/master@{#454609}
12 files changed