Add a histogram for Blink decoded image type

Add a histogram to record decoded images in Blink by type.

Notes:
(1) Both ICO and CUR images are categorized as ICO type in
the histogram since both use the Blink::ICOImageDecoder,
which does not discriminate by type.

(2) We only count visible images. In other words, 1x1 images
aren't counted.

Recording of stats implemented here: http://crrev.com/1242263011

BUG=513523

Review URL: https://codereview.chromium.org/1249273004

Cr-Commit-Position: refs/heads/master@{#341069}
1 file changed