Getter remains as it is needed on call sites. Setter can be removed.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/25e96490d3362bdcf07dbb28f5c481dab080409f

commit 25e96490d3362bdcf07dbb28f5c481dab080409f
Author: Reza.Zakerinasab <zakerinasab@chromium.org>
Date: Tue Aug 22 16:11:14 2017

Remove is_premultiplied_ from StaticBitmapImage

This change removes is_premultiplied_ from StaticBitmapImage and
its setter and uses the exposed SkImage::alphaType() instead.

Bug:  734601 
Change-Id: I2b44d5053ad959801dd32f2879548c0801a9ed8e
Reviewed-on: https://chromium-review.googlesource.com/619042
Commit-Queue: Mohammad Reza Zakerinasab <zakerinasab@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Justin Novosad <junov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#496320}
[modify] https://crrev.com/25e96490d3362bdcf07dbb28f5c481dab080409f/third_party/WebKit/Source/core/imagebitmap/ImageBitmap.cpp
[modify] https://crrev.com/25e96490d3362bdcf07dbb28f5c481dab080409f/third_party/WebKit/Source/platform/graphics/StaticBitmapImage.h
[modify] https://crrev.com/25e96490d3362bdcf07dbb28f5c481dab080409f/third_party/WebKit/Source/platform/graphics/UnacceleratedStaticBitmapImage.cpp
[modify] https://crrev.com/25e96490d3362bdcf07dbb28f5c481dab080409f/third_party/WebKit/Source/platform/graphics/UnacceleratedStaticBitmapImage.h