[canvaskit] Add demo excercising lots of text
[canvaskit] Add demo excercising lots of text

There are two approaches taken - naively using a bunch
of drawTexts and using drawGlyphs. The latter runs about
twice as fast (45 ms per frame vs 90 ms per frame) at the
expense of more complex code (need to layout individual
glyphs in the JS).

Change-Id: I5bc93048c0297f036215f86244b395cf812226a4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/502369
Reviewed-by: Nathaniel Nifong <1ce561679465ea4956c6210637580cd7ba456816@google.com>
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>

google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia