Half float fix

While running layout tests on MacOS, one of the tests failed within
CopyTextureCHROMIUM() because of an unsupported type. The origin of
the failure was wihtin canvas_color_params.cc, where GL_HALF_FLOAT
was used instead of GL_HALF_FLOAT_OES. The validators for this type
is initialized within gpu/command_buffer/service/feature_info.cc
and contains GL_HALF_FLOAT_OES, but not GL_HALF_FLOAT, which isn't
supported on GLES2 and unfortunately has a different value.

Bug: chromium:757974
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: If7a28c53db4a4f6acef035ee432153cec4f0acbb
Reviewed-on: https://chromium-review.googlesource.com/1057360Reviewed-by: Justin Novosad <junov@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#558379}

third_party/blink/renderer/platform/graphics/canvas_color_params.cc

@@ -210,7 +210,7 @@ GLenum CanvasColorParams::GLType() const {
     case kRGBA8CanvasPixelFormat:
       return GL_UNSIGNED_BYTE;
     case kF16CanvasPixelFormat:
-      return GL_HALF_FLOAT;
+      return GL_HALF_FLOAT_OES;
     default:
       break;
   }