Sync: MutableEntry shouldn't track changes that have no actual impact on the stored data

Sync: MutableEntry shouldn't track changes that have no actual impact on the stored data

This change optimizes MutableEntry and ModelNeutralMutableEntry
methods that update sync data. Most of these methods compare
new and old values and skip the update when the values match.
However most of these methods still unconditionally register EntryKernel
to be tracked in EntryKernelMutation list by making a TrackChangesTo
call, which immediately makes a deep copy of EntryKernel.
Then there is one more deep copy at the end of the
transaction scope.

This fix avoids the two unnecessary deep copies by calling
TrackChangesTo only when necessary e.g. when the new value
is actually different.

In addition I used this as an opportunity to do a small code cleanup around
marking MutableEntry instances as dirty.

BUG=468144
Committed: https://crrev.com/193873c7e382f11bf09b6739d07ac82e9dd14fd7
Cr-Commit-Position: refs/heads/master@{#339162}

[stanisc, 2015-07-14 00:20:42]

Patchset #1 (id:1) has been deleted

[stanisc, 2015-07-14 20:15:45]

stanisc@chromium.org changed reviewers:
+ pavely@chromium.org

[stanisc, 2015-07-14 20:24:33]

PTAL!

[pavely, 2015-07-15 20:13:12]

lgtm

[stanisc, 2015-07-15 21:31:02]

The CQ bit was checked by stanisc@chromium.org

[commit-bot: I haz the power, 2015-07-15 21:32:12]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1237493002/20001

[commit-bot: I haz the power, 2015-07-15 21:52:05]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-07-15 21:52:06]

Try jobs failed on following builders:
  mac_chromium_compile_dbg_ng on tryserver.chromium.mac (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.mac/builders/mac_chromium_comp...)

[stanisc, 2015-07-16 22:17:08]

The CQ bit was checked by stanisc@chromium.org

[commit-bot: I haz the power, 2015-07-16 22:17:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1237493002/20001

[commit-bot: I haz the power, 2015-07-16 23:19:05]

Committed patchset #1 (id:20001)

[commit-bot: I haz the power, 2015-07-16 23:20:02]

Patchset 1 (id:??) landed as
https://crrev.com/193873c7e382f11bf09b6739d07ac82e9dd14fd7
Cr-Commit-Position: refs/heads/master@{#339162}

[loyso (OOO), 2015-07-17 00:57:11]

A revert of this CL (patchset #1 id:20001) has been created in
https://codereview.chromium.org/1237843003/ by loyso@chromium.org.

The reason for reverting is: Causes crash
[19596:1287:0716/172955:FATAL:mutable_entry.cc(260)] Check failed:
kernel_->is_dirty(). 

in Builder Mac10.9 Tests (dbg) 


https://sheriff-o-matic.appspot.com/chromium/failure/sync_integration_tests%2....