Add missing null check in PageSerializer::serializeCSSStyleSheet()

Add missing null check in PageSerializer::serializeCSSStyleSheet()

Add missing null check in PageSerializer::serializeCSSStyleSheet() for
importRule->styleSheet(). CSSImportRule::styleSheet() can return null but
the function it is being passed to expects a non-null pointer.

Also update PageSerializer::serializeCSSStyleSheet() to take its CSSStyleSheet
argument by reference, instead of pointer to make it obvious to the caller
that passing null is not OK.

R=morrita@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176123

[Inactive, 2014-06-11 23:45:00]

This is the avoid a crash we've seen when saving http://na.leagueoflegends.com/.

Unfortunately, I haven't been able to add a unit test to cover this. I am able
to exercise this code path by adding a test to PageSerializerTest.cpp. However,
I wasn't able to get CSSImportRule::styleSheet() to return null.

[abarth-chromium, 2014-06-13 17:35:29]

lgtm

[commit-bot: I haz the power, 2014-06-13 17:36:33]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/ch.dumez@samsung.com/332573003/1

[commit-bot: I haz the power, 2014-06-13 18:41:42]

Change committed as 176123