Make SdchDictionaryFetcher free more memory when done.

Make SdchDictionaryFetcher free more memory when done.

It was just calling std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853
Committed: https://crrev.com/3bbfea923d0edbbde05223076f969ac6a44ba234
Cr-Commit-Position: refs/heads/master@{#400464}

[mmenke, 2016-06-17 17:24:11]

Patchset #2 (id:20001) has been deleted

[mmenke, 2016-06-17 17:57:06]

mmenke@chromium.org changed reviewers:
+ xunjieli@chromium.org

[mmenke, 2016-06-17 17:57:06]

Helen:  I went with the unique_ptr + reset() approach because we don't store the
whole dictionary in memory - the response body has bonus SDCH headers before the
vcdiff body, and we cut off the headers before storing.

Experimentally, this removes the the extra chunk of allocated memory I was
seeing stick in the SdchDictionaryFetcher.

[mmenke, 2016-06-17 18:11:29]

Description was changed from

==========
Make SdchDictionaryFetcher free memory when done.

It currently just calls std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853
==========

to

==========
Make SdchDictionaryFetcher free more memory when done.

It was just calling std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853
==========

[xunjieli, 2016-06-17 18:26:30]

I talked to Matt offline and understood this change better. 
This is solution (3) mentioned in crbug.com/620853.

LGTM.

[mmenke, 2016-06-17 18:33:59]

The CQ bit was checked by mmenke@chromium.org

[commit-bot: I haz the power, 2016-06-17 18:34:20]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/2075033003/40001

[commit-bot: I haz the power, 2016-06-17 18:39:12]

Description was changed from

==========
Make SdchDictionaryFetcher free more memory when done.

It was just calling std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853
==========

to

==========
Make SdchDictionaryFetcher free more memory when done.

It was just calling std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853
==========

[commit-bot: I haz the power, 2016-06-17 18:39:13]

Committed patchset #2 (id:40001)

[commit-bot: I haz the power, 2016-06-17 18:41:36]

Description was changed from

==========
Make SdchDictionaryFetcher free more memory when done.

It was just calling std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853
==========

to

==========
Make SdchDictionaryFetcher free more memory when done.

It was just calling std::string::clear() on the string
it downloads dictionaries to when a fetch is complete, which
may not free the memory taken up by the string. As the fetcher
exists for the lifetime of the Profile, and this buffer can be
a couple hundred k, this can waste a fair bit of RAM.

BUG=620853

Committed: https://crrev.com/3bbfea923d0edbbde05223076f969ac6a44ba234
Cr-Commit-Position: refs/heads/master@{#400464}
==========

[commit-bot: I haz the power, 2016-06-17 18:41:37]

Patchset 2 (id:??) landed as
https://crrev.com/3bbfea923d0edbbde05223076f969ac6a44ba234
Cr-Commit-Position: refs/heads/master@{#400464}