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This CL cleans up the code in said class and applies Chromium style.
vp9_reference_frame_vector.h file:
- Removes a a prescriptive class comment, instead uses a
descriptive one.
- Removes closed TODO from class documentation.

vp9_reference_frame_vector.cc file:
- Uses =default for destructor instead of {}
- Uses a std::bitset<> instead of bit shifting and testing,
which is error prone and more verbose.
- Uses std::fill() instead of an explicit for loop.

Comments are supposed to be in descriptive style ("uses")
and not imperative ("use"):
> These comments should open with descriptive verbs in the indicative mood ("Opens the file") rather than verbs in the imperative ("Open the file").

https://google.github.io/styleguide/cppguide.html#Function_Comments

Also fixed orthographic and grammar mistakes where applicable.

Change-Id: Ie78e2fbad5f3317e2c291127ac678360ce96d625
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207749
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771134}