Rename --enable-webapk command line flag to --enable-improved-a2hs

The new name is more consistent with the new description in chrome://flags
introduced in https://codereview.chromium.org/2485513002/

BUG=None
R=dfalcantara, isherman
TBR=sievers For comment change in ChildProcessServiceImpl.java

Review-Url: https://codereview.chromium.org/2496433002
Cr-Commit-Position: refs/heads/master@{#431602}
6 files changed