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On Android, the user data dir in Chrome and WebLayer by default is
ANDROID_APP_DATA_DIR. In the context of Java browsertests, this
directory is set to the isolated test root, which is problematic as it
can't be cleared between runs (see linked bug). This bleedthrough of
state can result in cryptic test failures.

//chrome's android_browsertests handles this issue by overriding the
user data directory in C++ at the //chrome level
(https://chromium-review.googlesource.com/c/chromium/src/+/1677591).
However, we are now facing this issue for weblayer_browsertests.

This CL introduces a general solution for this problem at the level of
NativeBrowserTestsActivity.java by allowing subclasses of this class
to specify the command-line switch for the user data directory. Doing
this specification will result in the user data directory getting
overridden to the embedder-specified private data directory, which
*is* cleared between each run by NativeBrowserTestsActivity. We do
this specification for //weblayer as well as Content Shell-based
browsertests. The latter solves the problem for the existing
content_browsertests and components_browsertests suites, which are
run on Android.

//chrome's ChromeBrowserTestActivity.java does not subclass
NativeBrowserTestsActivity as it subclasses ChromeTabbedActivity.
Changing //chrome's android_browsertests to use this mechanism rather
than its current one is left as future work.

Finally, we note that this solution is a stopgap, as it leaves open the
possibility that the C++ and Java sides have different views of the
conceptual "user data dir". The proper solution for this problem would
be for ANDROID_APP_DATA_DIR to *not* be set to the isolated test root
but rather to a directory that is cleared between each run (e.g., a
temp directory). That solution is however not easily accessible for
reasons detailed in the bug. In the meantime, the stopgap solution of
changing the user data dir on the C++ side only has been working in the
context of android_browsertests for ~18 months.

Bug: 617734
Change-Id: I8dd2fe25595f01a540af9a6c6c4787bb6570b62b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2504262Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Colin Blundell <blundell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#822575}