[Mac] Return an empty StringPiece from GetDataResource() when g_resource_data_pack is NULL.

[Mac] Return an empty StringPiece from GetDataResource() when |g_resource_data_pack| is NULL.

When running unit tests, |g_resource_data_pack| is never loaded. As a result,
if a resource is needed by code exercised by a test, the dereference of
|g_resource_data_pack| would previously crash, unless that resource id was
special cased.

This CL changes it to instead return an empty StringPiece, rather than
crashing. This allows PlatformBridge::loadPlatformImageResource() to
return a Image::nullImage() in this case, which is fine for some resources
loaded during unit tests.

In particular, this fixes a crash in webkit_unit_tests that would otherwise
happen with: https://bugs.webkit.org/show_bug.cgi?id=66707.

BUG=93397
TEST=Run webkit_unit_tests with patch from https://bugs.webkit.org/show_bug.cgi?id=66707. They shouldn't crash.

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98101

[tony, 2011-08-24 16:51:13]

LGTM.  Would it be easier to just add the image to the switch statement?  Either
way seems OK to me.

[Alexei Svitkine (slow), 2011-08-24 16:56:47]

On Wed, Aug 24, 2011 at 12:51 PM,  <tony@chromium.org> wrote:
> Would it be easier to just add the image to the switch statement?

I didn't go with this approach because that would require adding that
image to WebKit.

[tony, 2011-08-24 17:01:19]

On 2011/08/24 16:56:47, Alexei Svitkine wrote:
> On Wed, Aug 24, 2011 at 12:51 PM,  <mailto:tony@chromium.org> wrote:
> > Would it be easier to just add the image to the switch statement?
> 
> I didn't go with this approach because that would require adding that
> image to WebKit.

The image doesn't have to be in WebKit, you just need to make sure the image you
need is copied to the Resources directory in the app bundle.  I.e., you probably
need to update a 'copies' action in a gyp file somewhere.

[Alexei Svitkine (slow), 2011-08-24 17:08:25]

Fair enough. Either way, I prefer the NULL check for other reasons too:

- simpler code change (and no gyp change)
- faster unit tests (no need to read an image)
- will work for other such cases, where the image content is not
tested by unit tests (if it is, they'll still need to be special
cased, though)

[commit-bot: I haz the power, 2011-08-24 20:13:13]

Change committed as 98101