Refactoring in NaClProcessHost::DebugContext.  I use WeakFactory and so do not need to clear pointer inside DebugContext in NaClProcessHost destructor.
Refactoring in NaClProcessHost::DebugContext. 
I use WeakFactory and so do not need to clear pointer inside
DebugContext in NaClProcessHost destructor.

BUG= none
TEST= run_inbrowser_exception_test in nacl_integration

Review URL: http://codereview.chromium.org/9958114

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@130884 0039d316-1c4b-4281-b951-d872f2087c98

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