Android: Fix Proguard removing fragment constructors.

These started getting discarded when we switched to r8.
Two of these were previously fixed by adding @UsedByReflection,
but it seems safer to just keep all fragment constructors and
does not seem to keep anything unused.

Bug: 983904

Change-Id: I60bbc5a32dc49732aebd8048d674375cbc133f7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1703595
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Boris Sazonov <bsazonov@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678277}

base/android/proguard/chromium_apk.flags

@@ -15,6 +15,14 @@
   public static *** CREATOR;
 }
 
+# Keep all Fragment constructors since they are only usable via reflection.
+-keepclassmembers class * extends android.support.v4.app.Fragment {
+  public <init>();
+}
+-keepclassmembers class * extends androidx.fragment.app.Fragment {
+  public <init>();
+}
+
 # Don't obfuscate Parcelables as they might be marshalled outside Chrome.
 # If we annotated all Parcelables that get put into Bundles other than
 # for saveInstanceState (e.g. PendingIntents), then we could actually keep the

chrome/android/java/monochrome_public_bundle.proguard_flags.expected

@@ -117,6 +117,14 @@
   public static *** CREATOR;
 }
 
+# Keep all Fragment constructors since they are only usable via reflection.
+-keepclassmembers class * extends android.support.v4.app.Fragment {
+  public <init>();
+}
+-keepclassmembers class * extends androidx.fragment.app.Fragment {
+  public <init>();
+}
+
 # Don't obfuscate Parcelables as they might be marshalled outside Chrome.
 # If we annotated all Parcelables that get put into Bundles other than
 # for saveInstanceState (e.g. PendingIntents), then we could actually keep the