Avoid exception in console when changing base heap snapshot

Only existing provider should be disposed when disposing heap graph node to avoid the following exception:

An error happened when a call for method 'createAddedNodesProvider' was requested
TypeError: Cannot read property '(compiled code)' of undefined
    at Object.WebInspector.HeapSnapshot.createAddedNodesProvider (chrome-devtools://devtools/bundled/HeapSnapshot.js:1756:40)
    at Object.WebInspector.HeapSnapshotWorkerDispatcher.dispatchMessage (chrome-devtools://devtools/bundled/HeapSnapshotWorkerDispatcher.js:83:58)

BUG=None
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/141563012

git-svn-id: svn://svn.chromium.org/blink/trunk@166196 bbb929c8-8fbe-4397-9dbb-9b2b20218538