AW: fix magnifier icon color in dark theme

No change to logic. This adds a drawableTint attribute for the flag UI's
search icon, to ensure this is not too dark when the user has enabled
dark theme. This seems to have regressed due to
https://crrev.com/c/2389229.

Bug: 1135873
Test: manual - enable dark theme and inspect magnifier icon
Change-Id: I2d1f2ba245dbf8911cda10e8f09e94c7c137e255
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2455207Reviewed-by: Hazem Ashmawy <hazems@chromium.org>
Commit-Queue: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814943}

android_webview/nonembedded/java/res_devui/layout/fragment_flags.xml

@@ -34,6 +34,7 @@
             android:drawablePadding="4dp"
             android:drawableStart="@drawable/ic_devui_search"
             android:imeOptions="actionDone|actionSearch"
+            android:drawableTint="?android:textColorPrimary"
             android:textAppearance="?android:attr/textAppearanceMedium"/>
 
         <!--suppress HardcodedText -->