Make PrinterProviderAPI a pure interface

Make PrinterProviderAPI a pure interface

This makes PrinterProviderAPI a pure interface, with the
implementation hidden in printer_provider.cc, and with a custom browser
context keyed service factory. This will make it easier to inject a
fake API implementation in tests (e.g. for ExtensionPrinterHandler).

BUG=461114
TEST=extensions_browsertests --gtest_filter=PrinterProviderAPI

Committed: https://crrev.com/1378bc38c7b711ec75bcf38c07292326f051015d
Cr-Commit-Position: refs/heads/master@{#318124}

[tbarzic, 2015-02-25 20:35:15]

tbarzic@chromium.org changed reviewers:
+ rockot@chromium.org, vitalybuka@chromium.org

[tbarzic, 2015-02-25 20:35:15]

Mostly moving code around

rockot: owner review for gypi file

[Ken Rockot(use gerrit already), 2015-02-25 20:39:59]

lgtm

[Vitaly Buka (NO REVIEWS), 2015-02-25 20:55:13]

lgtm

[tbarzic, 2015-02-25 20:55:35]

The CQ bit was checked by tbarzic@chromium.org

[commit-bot: I haz the power, 2015-02-25 20:57:05]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/922833004/60001

[commit-bot: I haz the power, 2015-02-25 21:41:35]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2015-02-25 21:42:43]

Patchset 4 (id:??) landed as
https://crrev.com/1378bc38c7b711ec75bcf38c07292326f051015d
Cr-Commit-Position: refs/heads/master@{#318124}