Bug 1435456 - Remove asserts checking that downscale-on-decode is enabled. r=tnikkel

These asserts are somewhat faulty given the
image.downscale-during-decode.enabled preference is a live preference
and thus can change at any time. Given the decision to downscale is made
on the main thread, and it is asserted on a decoder thread, this will
always be inherently racy. Most of the time this isn't a problem, but
with our automated tests, we frequently flip this preference, and the
assertion may fail unnecessarily with an unrelated image. The reftests
themselves verify downscaling did or did not occur based upon comparison
to the reference, and don't require the assert for verification.

image/Downscaler.cpp

@@ -27,8 +27,6 @@ Downscaler::Downscaler(const nsIntSize& aTargetSize)
   , mHasAlpha(true)
   , mFlipVertically(false)
 {
-  MOZ_ASSERT(gfxPrefs::ImageDownscaleDuringDecodeEnabled(),
-             "Downscaling even though downscale-during-decode is disabled?");
   MOZ_ASSERT(mTargetSize.width > 0 && mTargetSize.height > 0,
              "Invalid target size");
 }

image/DownscalingFilter.h

@@ -98,10 +98,7 @@ public:
     , mInputRow(0)
     , mOutputRow(0)
     , mHasAlpha(true)
-  {
-    MOZ_ASSERT(gfxPrefs::ImageDownscaleDuringDecodeEnabled(),
-               "Downscaling even though downscale-during-decode is disabled?");
-  }
+  { }
 
   ~DownscalingFilter()
   {