Revert "implement SkPixmap::getPixel() with this->readPixels()."
Revert "implement SkPixmap::getPixel() with this->readPixels()."

This reverts commit 0944ae8afbafaae30eacffc73c5afbf4c062210c.

Reason for revert: Chrome roll?

Original change's description:
> implement SkPixmap::getPixel() with this->readPixels().
> 
> Yet another big old switch statement I don't want to maintain.
> 
> Once again, I think this at least makes it explicit that we're
> returning an SkColor in the colorspace of the pixmap.
> 
> Change-Id: I34aa7312c0a1670910586fc5a4b1951275e73e2e
> Reviewed-on: https://skia-review.googlesource.com/154864
> Reviewed-by: Brian Osman <brianosman@google.com>
> Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

TBR=14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com,brianosman@google.com

Change-Id: I827491cc047645876d5b7d5ac0067d266252f7ef
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Reviewed-on: https://skia-review.googlesource.com/155040
Reviewed-by: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>
Commit-Queue: Mike Klein <14574f09dfa9b4e14759b88c3426a495a0e627b0@google.com>

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