Return null ScaledImageFragment on failure of lazy decoding

This may be one of the causes of the crash when lazy-decoding huge
images. This CL by its own doesn't fix the crash, but avoids the
assert failure in LazyDecodingPixelRef::onLockPixels() about
bitmap.isNull(). We still need Skia to check for failure of
SkPixelRef::onLockPixels().

BUG=295895
TEST=ImageFrameGeneratorTest.decodingAllocatorFailure

Review URL: https://codereview.chromium.org/27279002

git-svn-id: svn://svn.chromium.org/blink/trunk@159718 bbb929c8-8fbe-4397-9dbb-9b2b20218538