Revert "remove half float workaround in GrGLGpu::onReadPixels"
Revert "remove half float workaround in GrGLGpu::onReadPixels"

This reverts commit 17ee85597a09757d1bdd31c4452bafcc430332f8.

Reason for revert: causing asserts on Chrome roll?

e.g., 

16:53:48.225 14315   0   Content Shell Framework             0x00000001107336dc base::debug::StackTrace::StackTrace(unsigned long) + 28
16:53:48.225 14315   1   Content Shell Framework             0x000000011065d4df logging::LogMessage::~LogMessage() + 223
16:53:48.225 14315   2   Content Shell Framework             0x000000011275ad01 blink::StaticBitmapImage::ConvertToArrayBufferContents(scoped_refptr<blink::StaticBitmapImage>, WTF::ArrayBufferContents&, blink::IntRect const&, blink::CanvasColorParams const&, bool) + 1361
16:53:48.225 14315   3   Content Shell Framework             0x0000000113e3f289 blink::BaseRenderingContext2D::getImageData(int, int, int, int, blink::ExceptionState&) + 777
16:53:48.225 14315   4   Content Shell Framework             0x0000000113b82a1a blink::V8CanvasRenderingContext2D::getImageDataMethodCallback(v8::FunctionCallbackInfo<v8::Value> const&) + 394
16:53:48.225 14315   5   Content Shell Framework             0x000000010e85b66e v8::internal::FunctionCallbackArguments::Call(v8::internal::CallHandlerInfo*) + 862
16:53:48.225 14315   6   Content Shell Framework             0x000000010e8598da v8::internal::MaybeHandle<v8::internal::Object> v8::internal::(anonymous namespace)::HandleApiCallHelper<false>(v8::internal::Isolate*, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::HeapObject>, v8::internal::Handle<v8::internal::FunctionTemplateInfo>, v8::internal::Handle<v8::internal::Object>, v8::internal::BuiltinArguments) + 1002
16:53:48.225 14315   7   Content Shell Framework             0x000000010e857c50 v8::internal::Builtin_Impl_HandleApiCall(v8::internal::BuiltinArguments, v8::internal::Isolate*) + 704
16:53:48.225 14315   8   Content Shell Framework             0x000000010e857758 v8::internal::Builtin_HandleApiCall(int, v8::internal::Object**, v8::internal::Isolate*) + 200
16:53:48.225 14315   9   Content Shell Framework             0x000000010f5b2a35 v8_Default_embedded_blob_ + 2212597
16:53:48.225 14315   10  ???                                 0x00000039bd68bcf1 0x0 + 247990893809
16:53:48.225 14315   11  Content Shell Framework             0x000000010f398946 v8_Default_embedded_blob_ + 8710
16:53:48.225 14315   
16:53:48.245 14281 [45/5600] virtual/gpu/fast/canvas/color-space/canvas-getImageData-rec2020.html failed unexpectedly (renderer crashed)

https://logs.chromium.org/v/?s=chromium%2Fbuildbucket%2Fcr-buildbucket.appspot.com%2F8944247412232349408%2F%2B%2Fsteps%2Fwebkit_layout_tests_on_Intel_GPU_on_Mac__with_patch__on_Mac-10.12.6%2F0%2Fstdout

Original change's description:
> remove half float workaround in GrGLGpu::onReadPixels
> 
> Change-Id: Ifb30a0b977918d84f5b9d5ea1714d19cc7392e37
> Reviewed-on: https://skia-review.googlesource.com/133440
> Commit-Queue: Brian Osman <brianosman@google.com>
> Auto-Submit: Brian Salomon <bsalomon@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>

TBR=bsalomon@google.com,brianosman@google.com

Change-Id: I16f0970e105e1068eb4976cd8fbb119e5830c087
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/133840
Reviewed-by: Stephen White <30b0ee0ef1651e8ba59cfd41f16e23abfe9aca8a@chromium.org>
Commit-Queue: Stephen White <30b0ee0ef1651e8ba59cfd41f16e23abfe9aca8a@chromium.org>

aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,aosp-mirror/platform_external_skia,google/skia,google/skia,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,HalCanary/skia-hc,Hikari-no-Tenshi/android_external_skia,google/skia,Hikari-no-Tenshi/android_external_skia,google/skia,rubenvb/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,rubenvb/skia,HalCanary/skia-hc,google/skia,aosp-mirror/platform_external_skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,rubenvb/skia,google/skia,google/skia,rubenvb/skia,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,rubenvb/skia,Hikari-no-Tenshi/android_external_skia,aosp-mirror/platform_external_skia,HalCanary/skia-hc,rubenvb/skia,HalCanary/skia-hc,google/skia,rubenvb/skia,google/skia,google/skia,HalCanary/skia-hc