Fix SiteInstance ref counting issue in RenderFrameHostManager::CommitNavigation.

Fix SiteInstance ref counting issue in RenderFrameHostManager::CommitNavigation.

RenderFrameHostManager::CommitNavigation was not using scoped_refptr when it created a new SiteInstance.  This resulted in the new SiteInstance being deleted in FrameTree::CreateProxiesForSiteInstance after iterating through the frame tree with base::Bind due to its ref count falling to 0.

This was the cause of crashes in  RenderFrameHostManagerTest.BrowserSideNavigationCrossSiteNavigation and RenderFrameHostManagerTest.BrowserSideNavigationIgnoreStaleNavigationCommit under --site-per-process.  These tests now pass.

BUG=417518
Committed: https://crrev.com/a175a7c2191008c6a03e63c451525e1e7ef3c437
Cr-Commit-Position: refs/heads/master@{#296584}

[alexmos, 2014-09-24 23:13:34]

alexmos@chromium.org changed reviewers:
+ nasko@chromium.org

[nasko, 2014-09-24 23:21:14]

Congrats on your first CL! LGTM

[alexmos, 2014-09-24 23:58:14]

The CQ bit was checked by alexmos@chromium.org

[commit-bot: I haz the power, 2014-09-24 23:59:24]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/599253002/1

[commit-bot: I haz the power, 2014-09-25 00:10:14]

Committed patchset #1 (id:1) as 2077ec214bcf42b1663a866e05ce1d12d2bc251b

[commit-bot: I haz the power, 2014-09-25 00:11:12]

Patchset 1 (id:??) landed as
https://crrev.com/a175a7c2191008c6a03e63c451525e1e7ef3c437
Cr-Commit-Position: refs/heads/master@{#296584}