ConnectionTypeObserver is being deprecated. This CL migrates
tab_loader_delegate.cc to using the replacement, NetworkChangeObserver.
NetworkChangeObserver's signals are less noisy because they come after network
connection has been established rather than while it is being established.

For more details, see the linked design doc in the bug below.

Bug: 754695
Change-Id: I335460ce0cd8e7c015b62bddc76c2f1d33f80bbc
Reviewed-on: https://chromium-review.googlesource.com/615460


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Helen Li <xunjieli@chromium.org>
Cr-Commit-Position: refs/heads/master@{#494448}