simplify GetRequestContext and GetResourceContext

simplify GetRequestContext and GetResourceContext

In BrowserContext, GetRequestContext is GetStoragePartition(this,
NULL)->GetURLRequestContext(). The rational is that this call should
call CreateRequestContext if needed, which would allow us to set
request_context_getter_ and return it. This patch makes it clear that
it's what's happening.

It also simplifies GetResourceContextEfl which was confusing.

Change-Id: I0b8109334a2ece8f053abc75fd9cb0b7eb0121d3
Signed-off-by: Arnaud Renevier <a.renevier@samsung.com>