Add histograms for animated image types

Add histograms for animated image types

Add histograms to compare the usage of image types used for animation.
Include the three we support (GIF, WEBP, and PNG).

In WEBP and PNG log the type once we know the size, since we are
guaranteed to know that the image is animated at that point.

In GIF, log the type once the start of a second frame is parsed, since
before that there is no way to know whether the image is animated.

In all three cases, an image will be logged twice, assuming that
deferred decoding is used and the image is decoded in the background
thread. (i.e. if the main thread decodes the size/frame count, but
the image is never decoded, the image will only be logged once.)

For PNG, this ignores images which were intended to be APNGs, but
includes images which are APNGs with only one frame, even though
they are not "animated" (such images are used to behave differently
depending on whether a browser supports APNG).

BUG=704425