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Previously, it would persist logs in uncompressed form. 
This CL changes the code to compress logs before they're 
saved. A new pair of prefs is introduced for storing 
these logs, while reading from the old pref is still 
maintained to not lose old logs. 

Additionally, this makes the metrics log discard limit 
(currently 50k) be checked against the compressed size 
rather than the uncompressed size. 

Simplifies the format used to store logs, now simply 
storing the compressed log bytes and corresponding hash 
for each log. The size and checksum fields are removed. 
(Size was redundant while the checksum is not no longer 
necessary now that we store the log hash.) 

Deletes some tests that inspected the actual pref 
values are removed, in favor of tests that check that 
serializing and de-serializing works as expected. 

Finally, also introduces GzipUncompress() that is 
needed for tests that inspect log manager logs. 

BUG=382076
TBR=agl@chromium.org

Review URL: https://codereview.chromium.org/318203004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@276429 0039d316-1c4b-4281-b951-d872f2087c98