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

Review URL: https://codereview.chromium.org/332573003

git-svn-id: svn://svn.chromium.org/blink/trunk@176123 bbb929c8-8fbe-4397-9dbb-9b2b20218538
WebPushClient requires a virtual destructor.

BUG=None

Review URL: https://codereview.chromium.org/338523004

git-svn-id: svn://svn.chromium.org/blink/trunk@176124 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Revert of Removing "using" declarations that import names in the C++ Standard library. (https://codereview.chromium.org/329183002/)

Reason for revert:
This patch is causing failures on the Chromium Linux Tsan v2 bot: http://build.chromium.org/p/chromium.memory.fyi/builders/Linux%20Builder%20(TSan%20v2)

The problem is that std::abs(int) is found in <cstdlib> but we apparently only include <cmath> everywhere. So the compiler only knows about std::abs(float) and variants.

You can probably fix this by including <cstdlib> in the places where you use std::abs.

Original issue's description:
> Removing "using" declarations that import names in the C++ Standard library.
> 
> This is as per the the coding guidelines for blink, which disallows the
> use of "using" declarations of any kind to import names in the standard
> template library.
> 
> BUG=NONE
> 
> Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=176032

TBR=tkent@chromium.org,ch.dumez@samsung.com,sunil.ratnu@samsung.com
NOTREECHECKS=true
NOTRY=true
BUG=NONE

Review URL: https://codereview.chromium.org/330933002

git-svn-id: svn://svn.chromium.org/blink/trunk@176125 bbb929c8-8fbe-4397-9dbb-9b2b20218538