Convert Vr tests to JUnit4

patch from issue 2857583005 (http://crrev.com/2857583005)

Convert Vr tests in chrome_public_test_vr_apk to JUnit4. For more detail,
please check src/testing/android/docs/junit4.md

Also removes the image tests from VrShellTest.java since:
1. They provide no actual test coverage since Render Tests don't actually fail currently AND the way they captured screenshots is pretty much useless for VR (just captures a black screen)
2. The move to JUnit4 seems to have messed with them - each call to renderAndCompare was taking an absurd amount of time (30+ seconds each).

BUG=640116

Review-Url: https://codereview.chromium.org/2876223004
Cr-Commit-Position: refs/heads/master@{#471965}

• chrome/android/BUILD.gn[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/VrShellNavigationTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/VrShellTest.java[diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/VrTestRule.java[Renamed from chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/VrTestBase.java - diff]
• chrome/android/javatests/src/org/chromium/chrome/browser/vr_shell/WebVrTest.java[diff]