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This CL is a smart-rebase-and-fix of the CLs below: it introduces
a new parameter |use_reduced_number_of_allocations_| to allow for the
new working mode described below and to temporarily circumvent the
GtsExoPlayerTestCases failures (b/121169667 and b/121003733); this
new flag is false when |output_mode_| is IMPORT, so all ARC++ cases
should work bc left untouched.

OTOH a mem savings overview can be found at https://goo.gl/3PaMiA.

Original CL description -----------------------------------------------
Vaapi decode: split |decoder_|s GetRequiredNumOfPictures()

This CL reduces the amount of PictureBuffers requested to be allocated
by the |client_| when we are not |decode_using_client_picture_buffers_|.
Instead, it "splits" the requested allocations into
- the actual needed PictureBuffers (A)
- the codec's requested reference frames (B) (a new method
 GetNumReferenceFrames() is added to AcceleratedVideoDecoder for this).

This splitting saves a lot of memory, since we allocate A+B buffers
instead of 2*(A+B). (B is 5 and A is 4-VP8, 4-12 H264/VP9)

Test: crosvideo changing resolutions for each codec, v_d_a_unittest
on nocturne (KBL) and caroline (SKL).

Bug: 912295

Reviewed-on: https://chromium-review.googlesource.com/c/1363807Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Commit-Queue: Miguel Casas <mcasas@chromium.org>
Cr-Original-Original-Commit-Position: refs/heads/master@{#615571}
Reviewed-on: https://chromium-review.googlesource.com/c/1379274
Cr-Original-Commit-Position: refs/heads/master@{#617366}
Change-Id: Ibf9a1455f8df4d52b77aee8e01f15c02878947ae
Reviewed-on: https://chromium-review.googlesource.com/c/1387391
Cr-Commit-Position: refs/heads/master@{#620025}