Allow zero inputs for feMerge

Allow zero inputs for feMerge

An feMerge which doesn't have any inputs should simply produce no output
(rather than failing the filter chain.)
Both FEMerge (w/ minor tweak) and SkMergeImageFilter are able to cope with
this case.
Any single-input primitives that sources from the merge will also not
produce any output. This behavior actually matches Gecko, and the spec
is not very outspoken on the matter.

BUG=533457
Committed: https://crrev.com/d3eeee8b67656cd6a031a0ea7abbe2a49fbc892b
Cr-Commit-Position: refs/heads/master@{#350884}

[fs, 2015-09-25 18:01:43]

fs@opera.com changed reviewers:
+ fmalita@chromium.org, pdr@chromium.org, senorblanco@chromium.org

[pdr., 2015-09-25 18:05:48]

The CQ bit was checked by pdr@chromium.org

[pdr., 2015-09-25 18:05:48]

lgtm

[f(malita), 2015-09-25 18:05:50]

lgtm

[commit-bot: I haz the power, 2015-09-25 18:06:53]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1368113002/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1368113002/20001

[Stephen White, 2015-09-25 18:10:28]

LGTM

[commit-bot: I haz the power, 2015-09-25 19:28:32]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-25 19:30:25]

Patchset 2 (id:??) landed as
https://crrev.com/d3eeee8b67656cd6a031a0ea7abbe2a49fbc892b
Cr-Commit-Position: refs/heads/master@{#350884}