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Makes the following changes to the VR browser tests to bring them more
in line with the Java implementation:

* Renames "webvr_*browser_test.cc" to "webxr_vr_*browser_test.cc"
* Adds some missing functions such as "PollJavaScriptBooleanOrFail"
* Adds failure messages to all asserts
* Standardizes function ordering
* Spelling/punctuation fixes
* Updates the Markdown documentation
* Renames the "vr_browser_tests" test suite to "xr_browser_tests"

Bug: 868143
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I4ec82eaed3101667cc7574fd0df81aa5d1ce155b
Reviewed-on: https://chromium-review.googlesource.com/1157697
Commit-Queue: Brian Sheedy <bsheedy@chromium.org>
Reviewed-by: Bill Orr <billorr@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581072}