TreatReturnedNullStringAs=undefined removed from XSLTProcessor.idl

TreatReturnedNullStringAs removed from XSLTProcessor.idl

XSLTProcessor::getParameter() returns value hold in hashmap 
m_parameters, the value it hold are set via setParameters()
and is set via JS bindings. 

The value of m_parameters key if reset or removed it returns
a null value.

getParmeter() is not based on W3C API but is compatible
solution with Gecko, with behaviour defined as Gets a 
parameter if previously set by setParameter. Returns null
otherwise.

Test in xslt-processor.html exhibits that value when is not 
set, reset or clear it returns in getParameter a null value
for the key. 

BUG=497982
R=philipj, haraken

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197109

[Habib Virji, 2015-06-11 09:13:27]

The CQ bit was checked by habib.virji@samsung.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-11 09:14:03]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174413002/1

[Habib Virji, 2015-06-11 10:27:27]

The CQ bit was unchecked by habib.virji@samsung.com

[Habib Virji, 2015-06-12 13:40:16]

The CQ bit was checked by habib.virji@samsung.com to run a CQ dry run

[commit-bot: I haz the power, 2015-06-12 13:40:37]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174413002/20001

[Habib Virji, 2015-06-12 14:27:53]

please take a look.

[commit-bot: I haz the power, 2015-06-12 15:00:11]

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

[commit-bot: I haz the power, 2015-06-12 15:00:12]

Dry run: This issue passed the CQ dry run.

[philipj_slow, 2015-06-12 19:26:06]

For the cases where the behavior has changed, can you also document the behavior
of IE and Gecko? If they also return the empty string, then it's clearly what we
should do, but perhaps they return null?

[philipj_slow, 2015-06-12 19:41:17]

I just took a peek at Gecko and what this API actually does. This shape of this
is a lot like setAttributeNS(), getAttributeNS() and remoteAttributeNS() and
it's pretty clear that if the parameter isn't found returning null is the right
thing to do. This is what Gecko does and it's necessary to distinguish an unset
parameter from one that was set to null.

In other words, make the return value nullable instead.

[philipj_slow, 2015-06-12 19:44:37]

I couldn't find a spec for this, but apparently it's a Mozilla thing:
https://lists.w3.org/Archives/Public/public-webapi/2006Mar/0021.html

Here it's explicitly documented as "Gets a parameter if previously set by
setParameter. Returns null otherwise."

[Habib Virji, 2015-06-15 08:37:26]

The CQ bit was checked by habib.virji@samsung.com to run a CQ dry run

[Habib Virji, 2015-06-15 08:37:37]

Thanks Philip for looking and also having a look in Gecko code. I have updated
it now to return nullable value.

[commit-bot: I haz the power, 2015-06-15 08:37:42]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174413002/40001

[philipj_slow, 2015-06-15 10:09:44]

lgtm

Awesome!

[commit-bot: I haz the power, 2015-06-15 10:39:59]

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

[commit-bot: I haz the power, 2015-06-15 10:40:01]

Dry run: This issue passed the CQ dry run.

[Habib Virji, 2015-06-15 11:23:45]

The CQ bit was checked by habib.virji@samsung.com

[commit-bot: I haz the power, 2015-06-15 11:24:03]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1174413002/40001

[commit-bot: I haz the power, 2015-06-15 11:28:13]

Committed patchset #3 (id:40001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197109