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If using canvas.getImageData or GL textures of UNSIGNED_BYTE type, 16-bit depth
stream data is available only through 8-bit* API, so there is a precision loss.
Here, we add no-precision-loss** JavaScript access through WebGL float texture.

RGBA32F usage here enables lossless access to 16-bit depth information via WebGL1.
In related work, the same code path is used to upload 16-bit data to other WebGL2
supported formats; e.g. with GL_R16UI there is no conversion needed in
SkCanvasVideoRenderer::TexImageImpl.

* 8-bit access refers to JS ImageData and WebGL UNSIGNED_BYTE where 16-bit depth
data is now available as luminance (all 3 color channels contains upper 8 bits
of 16bit value).

** Float is used for no-precision-loss access to 16-bit data normalized to
[0-1.0] range using formula value_float = value_16bit/65535.0.

This patch also adds UNSIGNED_BYTE WebGL test which was earlier tested through
testVideoToImageBitmap since UNSIGNED_BYTE and canvas rendering still share the
same path through SkCanvasVideoRenderer::Paint.

BUG=369849, 624436
CQ_INCLUDE_TRYBOTS=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

Review-Url: https://codereview.chromium.org/2476693002
Cr-Commit-Position: refs/heads/master@{#436221}