Clean up terminology used in about_flags.

Renames about_flags::Experiment to about_flags::FeatureEntry and
updates associated function names and comments. The goal is to
reduce confusion with the other meaning of "Experiment" in Chromium
- i.e. FieldTrial.

In about_ui code, tends to use "Experimental feature" terminology,
since it is more natural there, compared to FeatureEntry, which is
used internally by about_flags code.

Also:
  - Adds g_ prefix to global variables in about_flags.cc, so it's clear
    where they are coming from.
  - Some comment clean ups beyond just the simple terminology
    change.
  - Remove an extra include of data_reduction_proxy_switches.h,
    which was already included earlier.
  - Fixes AboutFlagsTest.NoSeparators, which previously only ever
    checked the first entry.
  - Removes IDS_FLAGS_UI_NO_EXPERIMENTS_AVAILABLE and
    IDS_FLAGS_UI_NO_UNSUPPORTED_EXPERIMENTS UI strings.
    In practice, these were never shown (there are always experiments
    and unsupported experiments), so the associated code/strings was
    simply unnecessary binary bloat.

BUG=490840
TBR=blundell@chromium.org

Review URL: https://codereview.chromium.org/1407753002

Cr-Commit-Position: refs/heads/master@{#354135}
11 files changed