[Instant Start] Remove unexpected toolbar shadow on start surface.

Demos:
Before this CL:
https://drive.google.com/file/d/1rGxWGyHhN1Nft2UQACOu6iH045dXhf7D/view?usp=sharing&resourcekey=0-iUuO4moaXQ1z4cqhbbzTdg
After this CL:
https://drive.google.com/file/d/1yjB2IVO0v7Kf2D---L3PVMhd_J4zbP9W/view?usp=sharing&resourcekey=0-Fyz_1WPerkvgv_2aBCVgVg

Bug: 1136245
Change-Id: Ia39231b975a12ec7f7739f428ba344d8005d5fcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2564010Reviewed-by: Xi Han <hanxi@chromium.org>
Reviewed-by: Matthew Jones <mdjones@chromium.org>
Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Hao Dong <spdonghao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833045}

chrome/android/features/start_surface/internal/javatests/src/org/chromium/chrome/features/start_surface/InstantStartTest.java

@@ -107,6 +107,7 @@ import org.chromium.chrome.test.util.ChromeRenderTestRule;
 import org.chromium.chrome.test.util.OmniboxTestUtils;
 import org.chromium.chrome.test.util.ViewUtils;
 import org.chromium.chrome.test.util.browser.Features;
+import org.chromium.chrome.test.util.browser.Features.DisableFeatures;
 import org.chromium.chrome.test.util.browser.Features.EnableFeatures;
 import org.chromium.components.user_prefs.UserPrefs;
 import org.chromium.content_public.browser.test.util.TestThreadUtils;
@@ -997,6 +998,43 @@ public class InstantStartTest {
         mRenderTestRule.render(surface, "singlePane_floatingTopToolbar");
     }
 
+    @Test
+    @SmallTest
+    @Restriction({UiRestriction.RESTRICTION_TYPE_PHONE})
+    // clang-format off
+    @EnableFeatures({ChromeFeatureList.TAB_SWITCHER_ON_RETURN + "<Study,",
+            ChromeFeatureList.START_SURFACE_ANDROID + "<Study"})
+    @CommandLineFlags.Add({ChromeSwitches.DISABLE_NATIVE_INITIALIZATION,
+            "force-fieldtrials=Study/Group",
+            IMMEDIATE_RETURN_PARAMS + "/start_surface_variation/single"})
+    public void testShadowVisibility() throws IOException {
+        // clang-format on
+        startMainActivityFromLauncher();
+        CriteriaHelper.pollUiThread(
+                () -> mActivityTestRule.getActivity().getLayoutManager().overviewVisible());
+
+        int shadowVisibility =
+                mActivityTestRule.getActivity().findViewById(R.id.toolbar_shadow).getVisibility();
+
+        Assert.assertEquals(View.INVISIBLE, shadowVisibility);
+    }
+
+    @Test
+    @SmallTest
+    @Restriction({UiRestriction.RESTRICTION_TYPE_PHONE})
+    // clang-format off
+    @EnableFeatures({ChromeFeatureList.TAB_SWITCHER_ON_RETURN + "<Study,",
+            ChromeFeatureList.START_SURFACE_ANDROID + "<Study"})
+    @DisableFeatures(ChromeFeatureList.INSTANT_START)
+    @CommandLineFlags.Add({ChromeSwitches.DISABLE_NATIVE_INITIALIZATION,
+            "force-fieldtrials=Study/Group",
+            IMMEDIATE_RETURN_PARAMS + "/start_surface_variation/single"})
+    public void testShadowVisibilityWithoutInstantStart() throws IOException {
+        // clang-format on
+        startMainActivityFromLauncher();
+        onViewWaiting(withId(R.id.toolbar_shadow)).check(matches(isDisplayed()));
+    }
+
     /**
      * Toggles the header and checks whether the header has the right status.
      *

chrome/android/java/src/org/chromium/chrome/browser/toolbar/top/ToolbarPhone.java

@@ -1488,6 +1488,7 @@ public class ToolbarPhone extends ToolbarLayout implements OnClickListener, TabC
         super.onAttachedToWindow();
 
         mToolbarShadow = (ImageView) getRootView().findViewById(R.id.toolbar_shadow);
+        updateShadowVisibility();
 
         // This is a workaround for http://crbug.com/574928. Since Jelly Bean is the lowest version
         // we support now and the next deprecation target, we decided to simply workaround.