IDL: initialize union member variables (to null/zero)

IDL: initialize union member variables (to null/zero)

Since we now use non-null to represent union member presence, either by
wrapping with Nullable<> or using the implicit null value (for pointers
and strings,) we must make sure the locals we pass to the implementation
are all properly initialized to null.

For strings and RefPtr<>/OwnPtr<>, this requires no special care due to
default constructors taking care of it. For RawPtr<> (when used in
practice, i.e. mostly in Oilpan builds) we need to initialize explicitly.

This fixes Oilpan crashes introduced by this CL:
  https://codereview.chromium.org/466323002/

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=180405

[Jens Widell, 2014-08-15 19:59:43]

PTAL

Note: This also affects code generation for attributes implemented in private
scripts (in PrivateScriptTest), also by initializing RefPtrWillBeRawPtr<> to
null explicitly. Not needed in that case, but hardly something we need to avoid
either.

[haraken, 2014-08-15 22:33:36]

LGTM

[bashi, 2014-08-16 00:12:25]

On 2014/08/15 22:33:36, haraken wrote:
> LGTM

LGTM

Thank you for the fix!

[Jens Widell, 2014-08-16 06:21:14]

The CQ bit was checked by jl@opera.com

[commit-bot: I haz the power, 2014-08-16 06:22:13]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jl@opera.com/474323002/1

[commit-bot: I haz the power, 2014-08-16 07:23:47]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  mac_gpu_triggered_tests on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/mac_gpu_triggered...)

[commit-bot: I haz the power, 2014-08-16 07:52:20]

Committed patchset #1 (1) as 180405