Fix for crash in Canvas2DLayerBridge related to graphics driver reset

After a GPU reset it may not be possible to obtain a graphics context
provider object.  What this means is that it is only safe to assume that
m_contextProvider is non-null when m_surfaceIsValid is true.

This change swaps the operands of logical operators in order to benefit
from predicate short-circuiting on m_surfaceIsValid, and it adds a
simple null pointer check.

BUG=366719,396153
NOTRY=true

Review URL: https://codereview.chromium.org/251493007

git-svn-id: svn://svn.chromium.org/blink/trunk@178695 bbb929c8-8fbe-4397-9dbb-9b2b20218538