Do we need a chrome flag here to enable users to set the hint text?

I'm guessing no. As long as testers can test it somehow, e.g. through command line flags.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5

commit ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5
Author: huayinz <huayinz@chromium.org>
Date: Wed May 31 21:21:02 2017

[Android] Omnibox Placeholder Experiment

Added:
+ Variations of hint text in omnibox placeholder for the experiment
+ Chrome flags to change hint text for testing purpose
+ Field trial testing config for testing purpose
+ The hint text change will show up on the second launch after
  flag is set.

BUG= 723908 

Review-Url: https://codereview.chromium.org/2903293002
Cr-Commit-Position: refs/heads/master@{#476027}

[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java/src/org/chromium/chrome/browser/ntp/NewTabPageView.java
[add] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java/src/org/chromium/chrome/browser/omnibox/OmniboxPlaceholderFieldTrial.java
[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java/src/org/chromium/chrome/browser/omnibox/UrlBar.java
[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java/src/org/chromium/chrome/browser/preferences/ChromePreferenceManager.java
[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java/src/org/chromium/chrome/browser/util/FeatureUtilities.java
[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java/strings/android_chrome_strings.grd
[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/chrome/android/java_sources.gni
[modify] https://crrev.com/ea3fb69a9bb9ce86d98fa6f9cabe6084a739fdf5/testing/variations/fieldtrial_testing_config.json

Quick question - can we make this string server-configurable? So we can experiment with new strings via Finch that aren't hardcoded into the client?

We can make this possible by overriding strings in Finch config. The only problem is Finch config does not fully handle localization. If we want the experiment to cover locales with different languages, we need to make a config file for each language and put the translated strings in each config file.

Yep that's fine. We can run experiments with specific locales vs. all locales.

Issue 664355 has been merged into this issue.