Measure usage of FormData.append(name, blob, filename)

Measure usage of FormData.append(name, blob, filename)

As requested by Anne van Kesteren:
https://bugzilla.mozilla.org/show_bug.cgi?id=1162658#c47

Per spec the blob argument cannot be null, but in Blink it can without
[TypeChecking=Interface], for FormData.append(name, null, filename).
(With no filename argument it is instead taken as the USVString "null".)
This is almost certainly safe to fix, but it's easy to measure here.

BUG=498790
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197268

[philipj_slow, 2015-06-10 15:18:42]

The CQ bit was checked by philipj@opera.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-10 15:19:04]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174973003/1

[philipj_slow, 2015-06-10 15:31:47]

philipj@opera.com changed reviewers:
+ sigbjornf@opera.se

[philipj_slow, 2015-06-10 15:31:48]

sigbjornf@, PTAL, I see you've poked at FormData a few times.

[commit-bot: I haz the power, 2015-06-10 16:35:29]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-06-10 16:35:31]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-06-15 07:59:46]

Friendly ping.

[philipj_slow, 2015-06-17 13:23:17]

philipj@opera.com changed reviewers:
+ jl@opera.com
- sigbjornf@opera.se

[philipj_slow, 2015-06-17 13:23:18]

jl@, PTAL.

[Jens Widell, 2015-06-17 13:32:33]

LGTM

[philipj_slow, 2015-06-17 13:40:32]

The CQ bit was checked by philipj@opera.com

[philipj_slow, 2015-06-17 13:40:32]

The patchset sent to the CQ was uploaded after l-g-t-m from jl@opera.com
Link to the patchset: https://codereview.chromium.org/1174973003/#ps20001
(title: "rebase")

[commit-bot: I haz the power, 2015-06-17 13:40:41]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174973003/20001

[commit-bot: I haz the power, 2015-06-17 16:03:44]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197268