Document some Android-specific quirks with PAC evaluation in Chrome and WebView.

Bug: 993499
Change-Id: I0d2f535ca6958d6e77ee9f854f30519a73ba57d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1912823
Commit-Queue: Eric Roman <eroman@chromium.org>
Reviewed-by: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714989}

net/docs/proxy.md

@@ -808,3 +808,33 @@ after finding the first candidate IP, so multiple IPs may be returned.
 Note that short-circuiting happens whenever steps 1-3 find a candidate IP. So
 for example if at least one IP address was discovered by checking routes to
 public Internet, only those IPs will be returned, and steps 2-3 will not run.
+
+## Android quirks
+
+Proxy resolving via PAC works differently on Android than other desktop Chrome
+platforms:
+
+* Android Chrome uses the same Chromium PAC resolver, however does not run it
+  out-of-process as on Desktop Chrome. This architectural difference is
+  due to the higher process cost on Android, and means Android Chrome is more
+  susceptible to malicious PAC scripts. The other consequence is that Android
+  Chrome can have distinct regressions from Desktop Chrome as the service setup
+  is quite different (and most `browser_tests` are not run on Android either).
+
+* [WebView does not use Chrome's PAC
+  resolver](https://bugs.chromium.org/p/chromium/issues/detail?id=989667).
+  Instead Android WebView uses the Android system's PAC resolver, which is less
+  optimized and uses an old build of V8. When the system is configured to use
+  PAC, Android WebView's net code will see the proxy settings as being a
+  single HTTP proxy on `localhost`. The system localhost proxy will in turn
+  evaluate the PAC script and forward the HTTP request on to the resolved
+  proxy. This translation has a number of effects, including what proxy
+  schemes are supported, the maximum connection limits, how proxy fallback
+  works, and overall performance (the current Android PAC evaluator blocks on
+  DNS).
+
+* Android system log messages for `PacProcessor` are not related to Chrome or
+  its PAC evaluator. Rather, these are log messages generated by the Android
+  system's PAC implementation. This confusion can arise when users add
+  `alert()` to debug PAC script logic, and then refer to output in `logcat` to
+  try and diagnose a resolving issue in Android Chrome.