GitLab

Observer interfaces having default-passthrough virtual methods rather
than pure virtual methods allows observers to only override functions
they'd make use of, and not have to override other functions.

This was inspired by: https://chromium-review.googlesource.com/c/chromium/src/+/2341407/33#message-8c9e7fa5cb0118798e3bd2f92bbe1cfe37a85640

Change-Id: I2e4e852204322a4608b4e449e0f7e9ab7e73f870
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2382959Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Darwin Huang <huangdarwin@chromium.org>
Auto-Submit: Darwin Huang <huangdarwin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#804474}