When tests fail, Chromium has some logic to extract the output of the failing test and make a short summary log that contains just the failure. Here's an example:
https://build.chromium.org/p/chromium.clang/builders/CrWinClangLLD%20tester/builds/35
https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.clang%2FCrWinClangLLD_tester%2F35%2F%2B%2Frecipes%2Fsteps%2Fbase_unittests%2F0%2Flogs%2FWrappedWindowProc.CatchesExceptions%2F0

However, we don't seem to have anything like this for compile steps. It would be enough for me to discard everything in a ninja build log before the first failing step, so the parsing logic would be to search for the first line starting with "FAILED:" and discard everything more than two lines before that.

As Chromium's build grows, compile logs get larger and larger, to the point that it takes several seconds to load a full compile log just to get to the interesting part at the bottom. Here is a recent ThinLTO OOM log that takes several seconds to load:
https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.clang%2FToTLinuxThinLTO%2F605%2F%2B%2Frecipes%2Fsteps%2Fcompile%2F0%2Fstdout

The interesting failure is on action [47595/47620], so there are ~50K lines of text and lots of buffering and "Loading streams..." before the interesting text loads.

Adding an error summary short log to the compile step would also eliminate the need for things like the "Buildbot Error" extension: https://chrome.google.com/webstore/detail/buildbot-error-2/hcjlifkoieegmpdmahecpeefinbnbpfa