SkRWBuffer: preallocate 'initialCapacity'

We're currently ignoring the hint, resulting in multiple unneeded
allocations later.

BUG=chromium:651698
R=scroggo@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2384763002

Review-Url: https://codereview.chromium.org/2384763002
1 file changed