Avoid redundant initialization of GraphicsContextState.

When creating a new GraphicsContextState the constructor initializes the state
with various default values. In GraphicsContext::realizePaintState we immediately
overwrite these values after creation. This patch adds a copy constructor to
GraphicsContextState to avoid that.

Even though this seems like a clear win, the benefits are not visible when
profiling. Looks like the overhead of the redundant initialization is
not very large. Still seems worth fixing though.

BUG=377687

Review URL: https://codereview.chromium.org/303613002

git-svn-id: svn://svn.chromium.org/blink/trunk@174985 bbb929c8-8fbe-4397-9dbb-9b2b20218538