[Sync] Speed up Javascript node operations

[Sync] Speed up Javascript node operations

Changed getNodeById to getNodesById and added getChildNodeIds.  This
avoids having to loop through a node's children in Javascript, which
is slow.

This speeds up expanding the "Autofill" node in the sync node browser
by 40-50% (with ~2700 elements).  With a debug Chromium, time spent
retrieving the nodes goes from 30s to 18s, and with a release Chromium
time it goes from 2.9s to 1.3s.

This will also speed up search, which will be implemented shortly.

Add timer logs to some sync node browser operations.

Fix docs for some functions called from Javascript.
BUG=76812
TEST=

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=86135

[akalin, 2011-05-19 23:50:40]

+zea for review

[Nicolas Zea, 2011-05-20 01:15:34]

LGTM