Add a new component (DownloadableStrings) to Chrome, that is
registered, on Chrome startup, if the
"DownloadableStringsExperiment" finch feature is enabled.

For the experiment's setup, start the download as soon as
possible to collect timings. More precisely, this will
Collect the following UMA metrics:

  - Download success / failure code.
  - Install time delta (on success) + timeout times (on failure).
  - Mobile Country Code (a.k.a. MCC) from the SIM operator,
    if available.

+ minor cleanups:

  components/crx_file/id_utils.h:
    - Add GenerateIdFromHash().

    - Remove unnecessary string allocations in is IdIsValid()
      and GenerateIdForPath().

    - Change GenerateId() to take a base::StringPiece instead of
      an std::string& parameter.

  components/update_client/utils.h:
    - Remove un-necessary string allocation + hex encoding +
      hex decoding in GetCrxComponendID().

R=sorin,waffles,jochen,jwd
BUG=766746

Change-Id: Idb0a132bf595864d0ea8d26fe9ad1cca3a9f5b63
Reviewed-on: https://chromium-review.googlesource.com/673147


Commit-Queue: David Turner <digit@chromium.org>
Reviewed-by: Jesse Doherty <jwd@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Reviewed-by: Sorin Jianu <sorin@chromium.org>
Reviewed-by: Joshua Pawlicki <waffles@chromium.org>
Cr-Commit-Position: refs/heads/master@{#515578}