Rename base::SmallMap to base::small_map.

Add a readme to base/containers to codify naming rules for containers.

Convert some uses of SmallMap to flat_map where appropriate.

Add an IPC serialization rule for flat_map (required by one of the SmallMap->flat_map conversions)

Improve the small_map documentation slightly.
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2825853002
Cr-Commit-Position: refs/heads/master@{#465911}