[chrome-devtools-frontend] Migrate to cloudstorage client

[chrome-devtools-frontend] Migrate to cloudstorage client

Import the cloudstorage library into the project to replace deprecated
'files' API.

Change pre_cacher module to populate local Cloud Storage server instead
of the cache.

file_reader module now relies on cloudstorage library's retries mechanism
instead of doing retries on its own.

TBR=pfeldman@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=295320

[mnaganov (inactive), 2015-05-20 00:06:28]

Committed patchset #1 (id:1) manually as 295320 (presubmit successful).