Implement setPlaceholderText method in UI.SoftDropDown class
Implement setPlaceholderText method in UI.SoftDropDown class

The new setPlaceholderText() method will allow to change the
title of a SoftDropDown when no item is selected and a
placeholder text needs to be displayed.

This is branched off from: crrev.com/c/1524645

Bug: 837323
Change-Id: Ibcf151bbbe0aa3bf8bf542e10fc82c607fa3f980
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1518924
Reviewed-by: Joel Einbinder <bab755abe9fee48fe544272e55f73517d4e4a725@chromium.org>
Commit-Queue: Hongchan Choi <69f024f858c7ec8905f5f0e21b2e518292ce6a82@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#656629}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 759e948d47bc575cd101aa2ddd6758c96b54bc98

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend