Fix WebstoreInlineInstaller ASAN bugs

Certain code paths which called back onto a raw pointer to a
WebstoreInlineInstaller could not guarantee the object's existence at
callback time.

Additional ref counts have been added to ensure that this is no longer the case.

This is a continuation of the reverted https://codereview.chromium.org/24839002/
due to a previously missed test case.

BUG=236513, 241431, 241432, 247774, 263004

Review URL: https://codereview.chromium.org/27034008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@228522 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 228522